[bladedtoys]

Like others have said, balancing play-ability is hard. Another problem is users may perceive the AI cheats which hurts fun.

But I'd say another hard thing is that every game needs a different approach to AI. Some need path finding, which is well documented. Others need resource balancing which is less so. The newer and rarer the game mechanic, the more novel the AI.

I would say Machine Learning is not useful except in a handful of games and if yours is not in that category, you risk spending a lot of time for little pay off. And even if in those categories, ML will be working to win not to maximize fun so they could end up being too good or to cheaty looking.