Hello 
We got Layla in 3.8 event! Very nice for those who didn't have her or for a constellation.
Unfortunately, she was and still is very underrated 
- Partially it's coming from very simple misconceptions regarding comparing her to Diona. Like:
X Diona is Tenacity of Millelith user
X Diona's shield is same / stronger than Layla's
X Diona and Layla are always choices for same role
Besides the last one, it's a matter of simple fact-checking, but for some reasons many people believe in things like this. We will take a closer look how it really is.
- Partially - from theorycrafters that often have strong bias against shielders
This also happened when Kirara was released. She was immediately underrated and doomposted by many of them. For example, opinion was being spread that her off-field dendro application is very bad and that she will need c4 for team like aggravate. For the most part opinions were fixed very quickly and some content creators admitted they were wrong. Luckily, Kirara is a popular fun dendro unit. Other characters weren't so lucky. Thus, they stay underrated. Unfortunately, Layla as well.
Let's try to fix this, let's consider her real value and compare to other shielders.
Layla - unique value 
Layla brings unique value by combining a very strong shield with an off-field cryo application in the form of homing missiles, what is great for reactions and getting buffs from Tenacity of the Millelith (ToM) or Deepwood Memories artifact sets.
In addition: good energy particle generation, small NA & CA buff coming with c4, DPS that can be meaningful, especially at c6.
Unique cryo application, vs Diona
- By her elemental application Layla can add reactions for many teams like superconduct, melt or freeze - that can help with survivability even more, but also with DPS by keeping enemies in place.
- If needed, it also can be easily avoided by simply not using burst.
- Meanwhile Diona doesn't have an off-field cryo application while her application can't be avoided if we want a shield or healing.
This difference makes Layla a unique unit opening new possibilities, and not just another cryo shielder to fill in the same role, as we will see in details.
Layla - best ToM user
It's a very good +20% ATK buff for the whole party, which can be stacked with the same buff coming from Noblesse Oblige set. In addition, a +30% shield strength bonus for party and +20% HP for Layla - to make the shield even stronger and also increases her DPS.
Why Layla is the best?
Because her skill hit enemies from off-field during it's duration which refreshes short 3s ToM buff. Also, its homing missiles form makes it more reliable.
For comparison:
- Zhongli - enemies need to stay close to his Stele to work
- Doesn't work - Thoma, Diona
Thoma and Diona don't have a skill that works off-field. That means only 3s of buff, from which you have to subtract time for switching characters and throwing skills. It might still be useful for getting high nuke DMG, especially with c6 Diona providing EM buff, but that's all.
Teams: 
- Freeze / Mono-Cryo
+ No only traditional Freeze, as Layla can work well also for Wanderer or Ayato.
+ In traditional Freeze with Ganyu or Ayaka going for second cryo character adding a lot to DPS is much recommended. But Layla still can be an option, might be with more DPS-focused build.
+ Layla is especially valuable for mono-cryo as she doesn't restrict mobility and there is enough room for more DPS-focused characters.
- Eula, Razor
Layla can be a good option and help with superconduct.
- Vapemelt - for Hu Tao, Yoimiya, Xiangling - national
+ Layla provides a nice increase in DPS by adding melts, especially combined with ToM and c4.
+ Limitations - not good for:
pyro characters with blunt attacks - Diluc, Klee, Yanfei, cause they trigger shatter first, not getting melts
enemies immune to freeze, because triggering freeze on them leads only to consuming hydro aura, thus - losing reactions for pyro DPS. It might work with double hydro, but still it's recommended to not use Layla's burst.
- Melt Quickswap
On DPS-focused build as a hybrid unit, with Benny adding healing.
- Melt Ganyu
Because Layla's cryo application can be avoided, Layla can be a good shielder not interfering with Ganyu's melts.
- Hyperbloom and Burgeon
Besides adding the advantages of freeze, cryo helps with keeping dendro aura - leading to more seeds. Especially in burgeon, where it helps with removing burning. It's an option to put the Deepwood set on her in those teams.
- Wanderer
He benefits a lot from a strong shield and a crit rate buff from his skill coming in contact with cryo, same with ToM and Layla's c4.
Shields - comparison 
From strongest to weakest in terms of raw durability:
- Zhongli
- Thoma - with all stacks
- Kirara - with all stacks
- Layla
- Diona
Note about other strong shields:
Yanfei c4 - is probably between Kirara and Layla, but may be higher, not sure. Besides requiring constellation, this shield is totally burst dependent with very high energy needs and not much utility besides that. It's usefulness is limited. I still recommend to look into it if you need pyro shield, but I won't be analysing it here. Noelle - she is a unit of different category, her shield is DEF-scaling and for herself, so she is out of this comparison.
- VS same element
If the shields of Thoma, Kirara and Layla are used against attacks of the same element, their durability can be higher than Zhongli's shield. It also might be in the case of Diona, but not sure.
- Layla vs Zhongli and Diona
The difference between Layla's and Zhongli's shields is usually somewhere between 1 to 2 and 5 to 8 in favour of Zhongli, of course. Very meaningful is also that Zhongli has easy access to HP% weapon - 3* Black tassel, while he don't have much need for other weapon, like - ER type. Meanwhile, between Layla and Diona difference is around 4 to 3 and 5 to 3 in favour of Layla. It all depends on constellations and investment. Interestingly, even on a more DPS-focused build with only one artifact with HP% main stat and 4pc ToM, Layla's shield still can be stronger than Diona's with a full HP build and Noblesse.
Shield - cooldown and reliability
Cooldown - why important?
Basically - shield is a barrier for enemy to break - in certain amount of time. Example just for understanding - if one shield is 10k HP and lasts 6s, while other is 20k HP and lasts 18s, that means effectively first shield can generate 30k HP during 18s, plus shortening downtime if broken. Duration, potential downtime, refreshing and other factors meaningful for reliability - all of that can be even more important than raw durability.
Zhongli has the best shield in other aspects as well. It doesn't require energy and has a long 20s duration that is much shorter than cooldown - 12s. Moreover, despite the slow shield casting, he can't be interrupted during the process.
Although Thoma has the second strongest shield, it only applies to its max potential coming from the skill and stacking mechanic of the burst. Shield from the skill alone is even weaker than Diona's. Thus, it's much burst reliant, which is combined with high energy needs. They are so serious that before c4 which helps in this regard, Thoma is much less recommended. Additionally, long cooldowns - 15s for skill and 20s for burst. All of that makes him much restricted to rotations and less reliable.
Kirara has 12s shield duration and amazingly - cooldown between 8-12s, longer if staying in her box form for longer time. This means Kirara not only has super strong shield, but also easy 100% uptime and quick refreshing if needed. That combined with nice mobility skill. No energy needed. That makes it second best shield in the game. Only limitation is - that it's much more for fast-paced teams where you can switch characters more easily. She barely competes with Layla, so we won't consider her kit more.
Layla - has a 12s shield duration and cooldown, which can mean from near 100% uptime to a few seconds of downtime in a rotation. For long rotations, there is an option to go for around 24s and using skill twice. Almost instant cast from skill, so energy is not needed.
Diona - shield from skill, no longer duration but worse downtimes, cause also coming from her kit:
+ press: 6s cooldown - 4.8s duration = 1.2s downtime
+ hold: 15s cooldown - 12s duration = 3s downtime
Casting issues:
- Shield generates only after hitting enemy with Icy Paws and hold version is slower, while Diona might get hit.
- Icy Paws are coming from behind Diona, what means they can hit an obstacle when standing close to one.
- Icy Paws for some reason hit dendro seeds.
Conclusion: Diona's shield is not only weaker, but also less reliable. But we have to remember that Diona is a shielder and a healer. We will consider value of it and more details below.
More about: Layla vs Diona
Different roles
Layla opened up new possibilities for some teams to have a shielder and off-field cryo application, like vapemelt, hyperbloom, burgeon, Wanderer freeze, Razor. Additionally, in hyperbloom and burgeon she can use Deepwood set much better than Diona. On the other hand - she can work without cryo application for vapemelt or Ganyu melt. Diona can't work like that.
Meanwhile, where their difference in cryo application doesn't matter much:
in standard Freeze - usefulness of both is limited, as it was mentioned, but Layla can be more worth it on more DPS build.
in Mono Cryo - there might be a higher need for mobility, while Diona's healing is limited to not the biggest AoE.
Stronger shield and more freeze vs weaker shield and healing
Let's not forget a shield is not only about survivability but also interruption resistance. It also protects against potential one-shooting, while healing - not at all. Finally - it can be stacked with damage reduction - like from Xingqiu's rainswords, making it even better. Taking all this into account, it's very good even if you are getting small downtime and taking some risk. It might be surprising, but playing with shielders also requires some learning and skills. Only Zhongli trivialize this so much it might be for the most part unnoticed.
Judging by themselves, I would say shields are more valuable than healing. That's why we also have mechanics against shields - Husk enemies generating their own if facing shielded characters or corrosion - that ignores the shields. In the end it's good to have both options.
Layla's role is straightforward - shielder, only adding freezing is one more thing to consider. While in case of Diona it's more complicated. Like - for Husk enemies she can be also problematic and we will have only her healing as an option. I would say that in many or most cases it's more worth it to go either for strong shield or for strong healing. If you want other option than Layla, then often better pick can be not Diona, but Kokomi or Qiqi. Yes, Qiqi - like for mono cryo or hyperbloom - providing MUCH better survivability and not restricting mobility. Similarly really good option might be Sayu - providing VV and leaving room for two / three characters focused on increasing DPS, not forgetting her mobility. Personally I haven't feel much need for combination of weaker shield with limited healing, but surely it's also valuable. It also can be a matter of characters and teams that player use and personal preferences.
Energy generation
Diona - frontloaded - especially good for Ayaka. But on the other hand, - in Freeze and Mono-Cryo Diona is not that much valuable
Layla - backloaded - over time - especially useful for Eula
It's often overrated how significant is Diona's energy generation is. It is very good, yes, but not that much better than from other cryo characters, including Layla. Also, if achieved through the Sacrificial bow, it means wasting time on non-DPS unit. In long rotations with Favonious and two skill uses Layla can even generate more energy particles.
Other advantages of Layla:
- Higher personal DPS, especially at c6
- Better buff and enabling: ToM, c4, superconduct, melt, fridge
- Even in the case of Noblesse Oblige Layla has better uptime than Diona
Mechanical issues
We've already taken a look at problems that Diona has with her skill when we were comparing shields. To this we have to add issues with her burst - it locks onto the position of the enemy on the cast, which can be problematic in many ways. For example, we can't throw it between/aside from the enemies, where we are trying to bait them / where we want to shoot from the a distance. It can also simply be missed if the enemy moves. Moreover - a number of times I've seen burst thrown in completely wrong, even inaccessible spots, no idea why.
Diona's bow
It's a utility in many ways, including hitting weak spots and having guarantee on fav proc or solving puzzles in exploration.
Smaller utilities
Diona provides a few special utilities but all of them are minor and even combined together are not very meaningful, including: enemy ATK debuff on burst from A4 passive, reduction in stamina consumption and movement speed buff from A1 passive. Still they provide something, can be stacked with other sources of similar effects and can be nice quality of life improvement.
C6 - hard to get, hard to utilize
Very often Diona is evaluated assuming c6 that provides better healing if the on-field character has less than 50% HP and +200EM buff, if more than 50% HP. But we are not in 1.x Genshin. Taking 4*s to c6 is hard and is only becoming harder. I have been playing for more than a 2 years and I still don't have her c6. Moreover, it doesn't even have that much usage. Diona's cryo application is problematic for Melt Ganyu, her healing - for Hu Tao, while in Hyperbloom and Burgeon Diona has issues with hitting seeds with her skill, while Layla can be more valuable thanks to freeze or as Deepwood set user.
Diona - very useful for beginners
Diona has amazing package of many utilities that can be replaced only by a number of characters. The same thing that makes her so valuable is also becoming problematic later. With her utilities, kit and scaling all over the place it's not easy to find aspecific role for her when the player already has more characters. In contrast, Layla's kit is very coherent with her HP scaling and focus on the shielder role.
Co-op, exploration and casual gameplay
Shield for co-op teammates: Layla from c1, Diona - c2. Besides that, I would say that in co-op Layla's cryo application is less valuable, while Diona's healing - much more. Similarly, with all her utilities Diona is more flexible for exploration. So, in the end, Diona can be very nice for more casual gameplay, far from min-maxing. All of that is not little and something to appreciate.
More about: Layla vs Thoma and Zhongli
Layla rarely would compete with Thoma. They can even work together - in Burgeon. Still, there are such cases, like for Wanderer. Both have pros and cons with Thoma having stronger shield on max potential, but with drawbacks. For vaporize teams I think Layla is clearly a better addition in most cases. She’s more helpful withreactions, rather than stealing vaporize from pyro main DPS.
Zhongli has an ultimate shield of Genshin. But... as it was mentioned, Layla is a more reliable ToM buffer and can provide a lot of value from her cryo application. She also has higher single-target DPS with lower field time requirements. Layla has the advantage of being more mobile compared to Zhongli using Stele for ToM, def shred, DPS and geo application. Finally, let's not forget that Layla's shield will be stronger against cryo attacks. In conclusion - in many ways and situations Layla can be a better option than Zhongli and simply best.
Shields value
Taking all above into consideration, I would rank the usefulness of shields in this order:
- Zhongli
- Kirara / Layla
- Thoma
- Diona
Conclusions 
Layla - one of the more useful units we got in 3.x Genshin
I'm not entirely sure why Diona is so popular and why so many people think that she is better than Layla, which is often supported by arguments that are simply false or misunderstanding. Maybe it's nostalgia because of how much helpful Diona was at the beginning - I have to say - also for me. Plus opinions from those that play a lot of co-op or casually, not caring too much about getting 36* stars in Abyss or high scores in events. To me it's very clear that Layla provides more value for harder content. Not only as a better option for the same purpose, but also as a character with broader usefulness, providing unique value and new possibilities to the game. But in the end all of those characters we considered have their own niches and advantages, including Diona. For the large part it also a matter of player's needs and preferences.
Layla DPS - FUN! 
Layla has a lot of possibilities for DPS playstyle, again, with her cryo application helping with superconduct and other reactions, plus - having a small shield for convenience. I highly recommend Feinerix YT channel very dedicated to Layla. In this video you can see Layla in a few versions: MOST SANEST LAYLA MAIN
Layla has something to tell you!!
Sources
KQM - the most reliable source, check if you want general info about Layla's builds and whatever you need!
Shield durability calculations
Thank you! 
Special thanks to Fren Gashishina for help with text editing <3
Share your own thoughts in the comments. Ask, if you have questions. Share if you play with Layla and in what teams.
