Yes it will be very hard to balance cavalry in SP and in MP. In medieval times numbers of cavalry units depended on part of Europe and time.

For example in 10 century cavalry was armored (most of them) with mail and there were in practise no units against it (spearman are good but they would fall after the charge). In middle Europe infantry was the main force but it changed in 15 century when Poland and Lithuanian used only cavalry units. In MP would for England (15 century) hard to stop cavalry army when they had small amount of horse units and anitcavalry units - historically speaking they would lost with polish army from 15 century (a lot of cavalry with many groups - crossbows, heavy, medium, with longbows -mongol improved Mongol standards). But to make game interesting it is needed to give every nation some of units than can counter enemy units.

The only ways to stop cavalry units are:
- small manoeuvre territory,
- well trained pikeman or something like them - morale of infantry was often weaker than cavalry,
- special barricades made before battle (and war wagons) - yes in game - this is huge plus for Med II.