Yeah, its true, the gravitational physics are a bit..... err.... posessed? :

If you kill somebody's horse who is charging into a hill, the rider (and the horse) would fall and slide all the way up the hill where they stop.
Likewise, if you jump your horse onto a steep hill, but he slides back, he takes massive damage when he reaches the initial ground, although he already landed on the hill, but just slid back.
On the other hand, I have jumped my horse from top of cliffs as high as I would say 7 storey building (if not higher) and he took just a little more damage than when he jumped into a 2 meter ditch.. In reality, horses would not take any hurt from jumping down 2 or 3 meters, however would most likely break their legs if they fall from 40meter high cliffs. And it is really annoying that when you jump your horse, it takes damage just because the ground you end up on is a meter or 2 below where you started ::\ doesnt make sense... One more thing, is that some maps have a 'death-traps' out of which no mounted unit can come out. When enemies get stuck in those, it could be very annoying. Lastly, water is another thing which acts like a 'death-trap' with the current physics in place, for it is nearly impossible for a mounted troop (I'm not saying players, since they are smarter than AI and would find a way) to come out of the water in a lot of places..