For me Combat AI is quite good. Enemies take cover, heal themselves, move, throw grenades...
Yes, enemies do quite a lot of different things. It's even as if they were designed to show off. Problem with this is two-fold:
1. You don't really see much of that beyond lvl 20. The brokenness of rpg mechanics kicks in and you start one-shotting or two-shotting everything. They don't have a chance to use all those "tricks." Even if those "tricks" made sense.
2. Those "tricks" don't make sense. They do them seemingly at random. Outside direct bumrush, if the algorithm tells them to move, there is no reason for it. They just run around without making any sense. You shoot one guy through the wall with Nekomata or Burya, and the other one who was just about to kill you runs for cover, and hunkers down where he can meet exactly the same fate. And gives you like 5 seconds to aim before he even starts shooting. Or they have 10 to 1 advantage plus they are already flanking you because you overlooked a guy or two - and they run for cover for no reason leaving the advantageous position. Or there are robots who bumrush by default, but human enemies do not try coop to flank you or overpower you, just stay where they are and hunker even though nothing is shooting at them. Funny stuff is when you miss a grenade toss, throw it somewhere else, but they elect to go there for no reason, vault cover with a fancy frontflip, and die (no reaction to grenades whatsoever). Ofc, it sometimes happens that they do what they are supposed to, and manage to get you with a seemingly clever maneuver. But those are 100% flukes.
Point is, there is a lot of stuff. It's just that none of that works or makes any sense. Like did you know there is a pretty cool instance of localized damage to the legs resulting in an awesome stumbling animation? I didn't. Until I farmed Skipper on a naked playthrough (without character bonuses gotta shoot those legs forever). Shooting legs to slow enemies down makes zero sense when you can just one-shot everything. Design of different elements is great, but the whole gameplay seems like it was put together last minute and never tested because the bosses wanted to release the game 2 years before it was ready.
So Doom Eternal where AI is quite simple for most enemies - all they do is they trade aggro tokens and aggro/bumrush with all they got - works infinitely better for gameplay.