So what ultimately killed my love for this game was finding out monsters ignore the AI in the wild. I was playing many games thinking the monsters would attack my enemies, only to be surprised when they did not.
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In 2.5.1 (and earlier also) monsters do attack the AI to some degree. Except for Tarth (at least units I think) which has a special trait to prevent this. But supposedly at higher (world?) difficulty setting monsters prefer the human player or sth - not sure it still works like that though. And I have seen weird things going on with monster attacks so from my POV its quite possible human player is treated special in general.
In addition to that AI only uses auto resolve for combat which unfortunately gives massively different results which can appear like cheating at times. For example I have seen some wimpy AI nobodies kill a healthy dragon. See here: Auto resolve battles makes my Assassin a superstar (There really should be an option to force tactical combat for AI vs AI. And if in player vision it could be optionally watchable too.)
I see there was a patch with multiple "AI updates" but not seeing specifics.
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Based on my modded playing the AI is very bad in 2.5.1.
So tempting to reinstall... ...haven't played any game in years.
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There appears to be a new bug causing AI controlled summon spells to malfunction in tactical combat.