I believe based on another thread that monsters never target Outposts, human or AI, although they may occasionally move into one at random. I believe not having the AI aggressively clear out lairs was a design decision to leave some for the player to fight later in the game, I don't know if the monster aggression against the AI was toned down to compensate for this. Certainly on higher difficulty levels the mobile monsters actively attack me, and if strong enough will head straight to attack my city, and I don't think they do that to the AI.
Monster behaviour has changed quite a lot in the last couple of patches, I haven't had that much chance to observe. Certainly lair monsters are much less likely to leave their lairs. They will apparently ignore cities more than one tile away. A strong mobile monster stack deliberately targeted one of my cities with weak defenders. On the other hand weaker stacks seem to wander past my city without attacking. I get the impression that mobile monsters are patrolling around a certain point, they seem to regularly return to that point. I don't know if they have a preference for certain types of location; for example in my current (Insane) game a Fallen Juggernaut has set-up camp next to a road. If it spots one of my units it will deliberately target it.
I believe based on the previous thread that monsters do treat the AI factions differently, certainly on higher difficulty levels. I'm not sure about Challenging. If:
I've seen a pack of wolves raze an AI city, I've seen AI scouts, pioneers and champions get whacked by roaming monsters (I've seen the animated combat on the strategic map) and I have seen AI players attacking (weak) monsters quite often.
then clearly the monsters kill the AI sometimes. I don't know if mobile monster movement is essentially random on lower difficulty levels, becoming more focused on the player at higher difficulty levels; that is certainly my hypothesis based on what Frogboy said in this thread: