I did auto-run for 12x8 map, setting epic, everything dense/plenty except resource being moderate. all stock factions plus Capitar and Umber custom sovs. Difficulty is Challenging/Challenging. After about 270 turns. I removed the fog of war.
In short, AI cheats on monster aggression. As I said, all AIs have semi-immunity for monsters.
More description : it cheats real hard.
Even more description : it is to the point of laughable and ridiculous (not really 'semi'... more likely 'almost'), and I wish I did not run auto-run, because after I saw how hard AI cheats on monster aggression, I completely lost any will to bother playing this game anymore until someone (either devs or modders) brings significant changes on this.
I suspect you won't notice much since most AI vs monster battle is mostly AIs attacking them first, and monsters really rarely attack anything.
A Magnar, with 250 power rating, with 9 cities completely surrounded by "strong" and "deadly" monster groups, from nearly 100 additional turns I only see 4 cities being razed, and by looks of it, it is most likely because the city was on the path of the monster movement and/or a city was built literally next to the monster lair. Pretty much all the time, monster movements inside of influence zone was nearly completely random, sometimes attacking unguarded pioneers but that's it.
And no garrison troops. Nope. Magnar's capital city, surrounded by three Earth Elemental armies, with just pioneer, not even fortress-type city (it's just a level 3 town.) Never gets attacked.
There is no need to believe or anything. Anyone who has this game check oneself. Do yourself a favor and do auto-run, and you will see. There is no need of arguing about player bias or something, because the evidence is just... right there.
Maybe, MAYBE it's because monsters overestimate garrison troops on city as Tuidjy said to me.... oh wait, all Magnar had was a pioneer unit on its capital city. Well, if those mightly elementals overestimate terrible city militia (level 3 town), well, all I say is all of these power rating thingly is completely FUBAR.
So, in order to comfirm Tuidjy's theory, I am running another auto-run, but this time world difficulty is "ridiculous" and AI is "hard". If I don't see any difference, I will try insane-novice. I will post the result later.