Just finished and won Expert/Expert on 12x8 map with epic setting, with all 8 stock AIs, playing my custom kingdom (Master Scout, Betrayers, Lightplate) with Beastlord. A while ago I also won Insane/Insane as Verga Yithril with similar setting, except the map was normal 'Large' and pace was 'normal' instead of 'epic'
Everything except resource frequency is 'dense' and/or 'plenty'. And here is my impression on AI.
1.) Monsters just do not attack AI as much as attacking players. Yes I did see an army of death demons attacking AI's invading stack from time to time, but it is ALWAYS absurd to see a very well-developed cities right next to slag/drake/dragon lairs. While they are nicer way to avoid 5-turn cooldown on 'raze' option for captured city (since as soon as I obtained a city, those huge monsters immediately attack that city so I don't have to bother waiting for 'raze' option to be available.)
2.) Super insta-fast constructions. I see someone already posted load-then-ai-instantly-finishes-wonder bug, but not only that, too many times I see AI finishes several-queued buildings/troops in just one turn.
Probably this is the reason that AI usually has 6x to 20x of research done even if I had twice more cities than he has. Compared to cities they have, they progress way too fast, even if 'Expert' gives them so much gold. I am talking about epic setting where rushing a pioneer costs about 300 gold to do so. This has to be fixed, otherwise as soon as AI gets better enough to use global magics, playing anything beyond 'normal' difficulty would be as if I am playing a roguelike game.
3.) Not only monsters do not attack AI much in general, AI itself has hard time 'clearing' its area.
4.) pioneer AI uses is not one-time unit like player's. They do expire when building an outpost, but I always see their pioneer still alive after a city is built. This also leads super easy expansion.
5.) Not sure it is finally fixed, but AI should not get 2 free essences when a city is built. Due to I did not have Stealth for the recent Expert play, but when I played as Verga, too many times I saw AI built a city on tile without essence, and doh! two essence just appeared on that city from nowhere!
So, basically, -I am not sure about lower difficulties- AI has hp boost, better starting sov (they have free horse if you play any difficulty that states 'AI sov gets slightly better'), gold boost, insta-completed construction, better Stealth (since player's Stealth does not protect cities now, also it does not work on wildland monsters. For AI, both work well) and cheating on city tile (I keep want to say restricting where I can build cities is just cheap idea to try to prevent city spam, and in reality it does not work and only causes frustrations IMO).
I understand HP boost, better sov, more gold, and free essence slots. But Monster semi-immunity and insta-build needs to go, from ALL difficulties. Here is my reasoning.
1.) Monster semi-immunity really breaks immersion of this game. I worked hard to get rid monsters from my area, only to see AI having nearly no problem keeping their cities AND improvements (yes, it seems while my improvements keep getting invaded all the time, AI's improvements are all advanced and seems was never attacked despite it was right next to slag lair.) Not only fair, but it also makes 'Stealth' option near-useless for AI. (thus Tarth got a huge disadvantage with 2 bonus points wasted on them) Nerfing Stealth option while not fixing those AI's free, better Stealth ability makes no sense to me.
This really should be exclusive to Tarth or any faction with Stealth ability only, not all factions, on any difficulty.
2.) And yes, insta-done-construction bug.... Imagine when AI can use global magic, and/or able to recruit Ogre/Dragons.... Already bringing Dragons when my sov is barely level 6... such sweet dreams. Not to mention this bug seems unstable and unpredictable, I'd say they shouldn't allow to do it in the first place.
At last but not least, a very big thanks for nerfing Beastlord again despite it was nerfed a while ago. With recent changes on starting position, I see a option to play as Beastlord is now near-impossible for any high difficulties. I just don't like nerfing things here and there.... and we have already VERY few limited option to play harder difficulties already.
Please, at least bring back a way to use monster lairs (like we could for Naga and Igni during beta 3 or earlier) for Beastlord. And maybe allow Beastlord to tame Elemental as well (tho it may be non-sense, and it should be belong to Summoner.)
Now, I will play Beastlord again, but this time I will get Stealth+Master Scout and I will lower the difficulty (Challenging) see if AI behavior changes.