Normal -> Challenging -> Hard -> its easyeasyeasy. Small, Medium, Large maps, whatever.
Ridiculous (on the previous builds) was actually difficult - if 900% production bonuses can be qualified as difficult
The AI isn't really fully functional yet. It can 'play' the game, but its vulnerable to any number of exploits - and there are so many aspects of the game that are exploitable right now it barely matters what you set it to.
Or to put it another way - no, the AI is not the reason people are complaining about everything from the heroes to the quests to the units to the tac combat to the spell system. There are major issues with just about all of the systems, some of which are glaringly obvious, regardless of difficulty.