As with most of my posts, I'm overly verbose, jump to the TL;DR for a summary or my meanderings...
Let's start with the obvious, AI is very difficult to program. Think about all the possible variables that are involved, remembering those variables need to be recalculated every fraction of a second based on an ever changing world. Sure there are libraries out there that will cut out a lot of the work, such as Unity's pathing algorithms but it's still needing to calculate a ton of stuff just to work out where one zombie should go, factor in a horde and factor in that the horde needs to act as a group of individuals rather than a uniform group of zombies (ie: all following exactly the same path/strategy/etc). Add in different attack methods, such as the cops or the spiders and you can see how the basic underlying logic can get complicated and that's before we've factored in the difficulty settings where each AI calculation will be based on how aggressive they are to be.
So, it's not easy, not easy at all. Not only is there a ♥♥♥♥ ton of calculations involved it also has to feed into the immersion so that zombies act like zombies - they aren't robots so should act as if they are sentient even if it is a brain eating, decaying sentience! It's because of this I cut a lot of slack with AI in games - anyone who has been playing for any number of years will have seen the level of AI improvement over the years (just go back and play some old games to see what I mean, AI was easily the weakest element)
Ultimately I'm not looking for uber-intelligent enemies but I do want ones that will challenge me, and not just because they can take out concrete and steel easily. Currently the AI is a challenge only if I put the difficulty at max (heh... that combined with my low FPS!) and even then it's not so much the AI that's challenging but the sheer brute force that they offer up.
7DtD suffers very much from CoD (not the FPS franchise but Circle of Death) where zombies will get caught up in their own little world and just run around in circles until interrupted by a timely arrow to the forehead. There is also the perma-fence jumping glitch where zombies see no problem with continually jumping against a seemingly weak wooden fence rather than simply plough through it - especially as it's more than capable of doing that it just chooses not to. There are a few other little glitches along the same lines. It's all too easy to fool the AI - especially when there is a height difference between you and them. So all in all, the current AI is more noticeable due to it's "glitches" than it's successes or the level of fear it can strike into the player.
TL;DR?
It's not so much that the AI should be better, although I believe it should, the first port of call would be to iron out the failings of the current AI. The running in circles, the getting stuck behind an obvious and easy obstacle, unwillingness to destroy something that they repeatedly try (and fail) to jump over. It's only once you remove those immersion breaking stupidity of the AI can you go on to improve.
Personally I'd like to see ridiculously hard AI, one that beelines onto you and beats on you like a ginger stepchild until your only option is to flee. "Why?", you ask. Because the current upper limit of difficulty should (imo) be almost impossible to deal with but it's really not. I'd much prefer hard as nails AI and need to dial it down, than limp wristed idiots which I need to dial up.
** I would love to see the return of the "smell detector", and perhaps even extend it, where you can be detected via fresh meat you carry, but either I missed it or it's not coming back for A17