Serene Grace is 1) a useful ability, 2) not broken/OP on its own, and 3) an ability that does have direct implications to helping our core and to helping counter Aegislash.
We're, for the most part, assuming that serene grace would work for CAP18 alongside scald/maybe lava plume, and for good reason; giving the typing, most water types have scald and a fair amount of fire types have lava plume. 80 BP moves with a 60% burn chance is nothing to scoff at. Fear of this being overpowered is ridiculous, however. Entei has a 100 BP 50% burn chance move, which is very much akin to these numbers. Entei is far from being a top-tier, meta-breaking threat. I don't think giving CAP18 a similar move-based presence will break it, either.
Both Lucario and Latias would appreciate having foes burned. Lucario likes the passive damage that burn offers, as it is akin to having stealth rock up after only one turn of burn residual damage for the particular inflictee. Latias appreciates making threats have lower attack to remedy the lower of her defensive stats.
Aegislash himself doesn't like burns, let alone fire STAB. However, 1 on 1, Aegislash is already likely to flee against our CAP18 and switch out. Serene Grace Burn Chances dissuade an opposing Aegislash from switching out to a potential counter in fear of the counter getting burned. This is almost akin to a pursuit factor of risk, though probably not as extreme. It does however intensify the risk/reward situation that an Aegislash user has when facing a serene grace burner.
Serene grace burns have an extremely major advantage over will-o-wisp in the respect that they allow offensive pressure alongside the defensive benefits that burns offer. In the earlier typing stage, we decided having a CAP18 that could offensively pressure foes was what we wanted to do. Serene grace allows the opportunity to keep this offensive pressure while also offering much needed defensive benefits to itself and its core members.
Regenerator allows CAP18 to switch in and out more, but has little impact on how much Aegislash will switch out against CAP18. Unaware allows switching in against physical sets easier, but offers no incentive for Aegislash to stay in and in all likelihood will let Aegislash switch out to a counter for a low-cost switch.
Having a method to burn and cripple Aegislash means we do not need the massive amount of bulk it would take to absorb shadow claws, or a single shadow claw assuming we can switch in while he sword dances. The biggest thing to fear would be a shadow claw on the switch and a shadow sneak before making a move. Likewise, bulletproof (mentioning for comparison sake--the post here is clearly focused on serene grace, not bulletproof) CAP18 fears the same thing. Serene grace also arguably allows for CAP18 to force to take a shadow ball, then a shadow sneak, but really this damage output would likely be similar to a pure physical shadow claw + shadow sneak combo anyway. If bulletproof allows CAP18 to invest more in physical bulk to take physical hits, I think serene grace would to a certain extent allow more special bulk to be allotted for taking shadow balls. The difference is serene grace means CAP18 would have to only take a single shadow ball of damage to reliably switch in (and possibly an uninvested shadow sneak before being able to move). Most mixed Aegi's do not run significant attack EVs, therefore CAP 18 would likely be able to take less damage from a mixed shadow ball + shadow sneak combo that from a shadow claw + shadow sneak combo. Furthermore, standard special Aegi doesn't even run shadow sneak and it would definitely be safer to take a single shadow ball than a shadow claw + shadow sneak combo. A serene grace CAP18 that is specially bulky will likely have to take only one special hit to switch in, whereas a bulletproof CAP18 that is physically defensive will have to take a shadow claw and a shadow sneak. Bulletproof would likely drastically change Aegi's to physical, but serene grace would likely not have as large of an effect. With bulletproof, we're asking for shadow claw + shadow sneak. With serene grace, it is likely shadow ball sets will still be run, and once a shadow ball set is identified, the switch in choice becomes easier (in theory). I fully admit that relying on keeping Aegislash to still run shadowball has some other flaws, and that it doesn't prevent shadow claw + shadow sneak variants from showing up either. But Serene Grace seems to have the potential to run special bulk to take a shadow ball and more actively spread burns to cripple physical Aegi's. Yes, w-o-w with another ability might accomplish this as well, but arguably with less utility because of the loss of offensive pressure that could also help out the team.