As the previous posters pointed out the AI usually just builds whatever it can in a given settlement often resulting very one sided armies.

Also, even where it could be possible the AI builds very few cavalry making it even more easier for the human player.

The only good stacks you are likely to encounter are the Mongol original stacks. These are not just very well balanced but consist of highly experienced soldiers led by experienced generals (this later is also missing for most AI armies).

Alas, once the mongols settle down they seem to revert to the "old AI" tactics of producing whatever they can. I have met a mongol army consisting of more than a dozen town militia and one or two light lancer.

The AI often hires mercenaries too, which is not much of a help, at least in europe as mercenary stacks are also rather one sided, consisting mostly of spears and shooters. The easiet way to help the AI would be to create well balanced mercenary pools, with spears, shooters, heavy and light cavalry (one would think that at least in eastern europe one should be able to find plenty of light cav mercenaries, like hussars but no).