Yeah, I think the AI was responsible for most of the delays and suffered the most cuts. The way the train station and car park in the garbage are constantly getting raided every time you go through those zones is dumb and it does spoil the immersion. I think the game could have done without that.
In other ways the AI can be really cool though, like when you see the dogs working in a pack and being bolder in greater numbers, or dragging bodies off to eat them. Or the boars chasing the dogs round. Or the packs of little rats, I've seen about 20 of them take out a group of 5 stalkers which was fascinating. Apart from the perpetual attacks in the garbage I mentioned above, I wouldn't say the AI were particularly scripted though. Of course some mission bits have to be scripted to a certain degree, but aside from that the game is still tracking a lot of AI moving round the game world on their own, which pretty much doesn't happen in most other games.