It also depends on the horse. Even with a hunter, charge in a group that is just a bit too dense, and you're stopped in it. And it happens to reduce a lot the health of your horse, then yours, once he's dead. Chargers (and warhorses) don't have this kind of problem, and will trample scores of ennemies like mere anthills, though.
The player is also easily capable of avoiding the lance's tip, and then smash his [type there the name of your weapon, preferably a havy one, like a great axe] in the face of the poor cavalryman. But not the IA, who is just a IA, of course.