I'm honestly usually annoyed by how Celestials are handled in general. I get they weren't given a lot of attention because they aren't supposed to be fought by the players, but it still leads to problems.
For example, a DM of mine just finished running us through a massive fight against an insane amount of devils (legit, if we were using expeirence, our level 13 characters should have ended up at level 25, it was insane). We ended up getting the aid of a Planetar, a Solar, and an Empyrean during the fight. Things that came up.
1) What happens when a celestial is killed on the mortal plane? Devils and Demons get sent back to their respective planes, but no where could we find any lore telling us what happens to Celestials. Do they just die as normal? Why? Why is it easier to kill a servant of the gods than it is to kill even the weakest of devils?
2) Anitmagic. It was insane, but the final battle had the Devils using a massive numbers of Antimagic fields. Turns out, per a straight reading. That a Pit Fiends fire mace is just a natural effect of their being, so it is unaffected. Angelic Weapons though make the weapons magical and add the radiant damage. So, in an Antimagic field they are far weaker than the Devils.
3) Lack of ranged options. Turns out, Planetars don't have a ranged attack. They can move 120 ft sure, but the common "Hurl Flame" attack of Barbed Devils and Horned Devils (both lower CR) are 150 ft range, so they can get kited for a few turns, as the devils just strike at them from a distance. Solars get their longbow... but they only get 1 shot a turn? They get completely outclassed at range
4) A lack of variety means they just can't keep up. So, Mordenkainen's released more devils. Including the Abishai, the Red and Blue Abishai being incredibly powerful (red's deal 14d10+1d8+18 if all their attacks hit, and Blue's have lots of big spells). The vast variety of the various demons and devils meant to fill different roles means that if I need a sneaky devil, or a fast devil, or a magic devil, or a powerhouse devil, or a cannon fodder devil. I got it, I can easily find it. Look at the Celestials. Devas and Planetars and Solars are good at healing, fast at moving, and powerful melee combatants. And their spell lists are nearly copy and pasted from each other. They can't respond to a variety of situations. In fact, they can't even fulfill one of their major literary functions. Only Deva' can disguise themselves as a mortal. You know, that thing that angels do constantly in literature so that people can prove they were good and then the angel can reward them? Not possible per RAW for Planetars and Solars. And, also, most of them just aren't interesting enough. Not only can they not do much that is interesting, but with only three covering all good Angels, you don't get a difference between them. Angels of love, war, crafting, knowledge, hunting, seas, storms, volcanoes... all the exact same.
