most was already mentioned, anyhow:
not enough focus on essential starting techs and the things they enable like markets, city states, education, sage
non-elf civs should beeline bronze working if there are many trees around
AI is over-expanding a lot. on noble it can afford some but it could do a lot better without spamming cities everywhere without having access to city states, courthouses, forbidden palace, summer residency, markets and kilmorph temples. it should be manageable to calculate the maintenance cost of a new city (including the heightened cost for all other cities) and decreasing the value of settlers accordingly.
no flexible defense. if the AI stacks units in it important cities you can take them out one by one instad of being overwhelmed by a combined assault.
war declarations with too small stacks. in my last game (submitted for ecofarms immortal thingy) i got early war declarations with only 5 warriors/scouts. they moved back and forth until i killed them as they had no chance attacking my city (at least they did not attack)
moving in stacks is not flexible, a single sun2 spell (binding light?) can immobilize 20+ units until you massaged them down with some air-magic
promotions are not handled well. at emperor and below a single shock2 warrior can eleminate whole civs (with the captured cities supplying).
as there are less paper/scissors/stone units (like spearmen, axes etc in vanilla) in FFH the AI should fill this gap using promotions.
having like shock2, formation and cover units to protect a stack would make it a lot stronger in defense, not speaking from attack.
(many a human player is not able to do this, too

)
AI does not manage healing. stacks should be supported by disciple units that are not used in combat - except maybe in 90%+ situations
heroes are not covered enough, you can often wear them down.
orthus should get some company and wait till 10xp at least. should go for combat1, shock, combat3, march, shock2.
strong barb units should not sit in cities defending them. especially horsemen. and orthus. and...
collateral units should only attack if there are sufficient other units available. bombard or withdraw else.