As the title stated, Trample does not interact well at all when the terrain has some kind of imperfection (although I'm not sure this is the right term to define exactly what I want to put out). I can't quite put my finger on it, but if you try to use Trample through some (certainly not all of them) doorways or archways, or when there is some kind glitched out terrain between you and the target, the horse will just immediately stop charging.
I'm trying to put up a list of situations where I encountered this, but there are some obvious ones that come to mind, like the end of cycle boss arena in the Infinite Archive, for example, where Trample sometimes encounters some kind of terrain imperfection and then immediately disappears.
What I want to say when I say "terrain imperfection": I mean this because shortly before disappearing, the horse sinks to the ground, as if there was something there, some kind of invisible hole in the mesh of the ground (that certainly should not make the horse stop charging, but it does). Notably, Trample works fine even in some mountainous terrain, where the horse charges downward, many times without issue, but it can't handle these "terrain imperfections".
Nor can't it handle many doorways and arches found in the game. This is not consistent, though. In cities, the horse seems to trample through doorways just fine. But when I'm in a dungeon and there is some kind of archway before me, the horse will just disappear and not complete its charge. I'm not entirely sure if it is related to archways, but this is just my running theory. I have compiled some screenshots of the dungeon where this issue happens the most (Spindleclutch II).
I really love the trample skill and I think it is already clunky enough to use in content that it should not have this kind of pathing issues. I'd be very interested to see a fix or some kind of workaround to make this skill work as it is supposed to.
My toon idle just some feet away from the Spindleclutch II entrance
Preparing to cast Trample on the bugged spot
Horse hits bugged spot and fades away
Casting trample at a slightly different angle to demonstrate that it reaches it's target (in this case, the NPC), at full range, without fading away or stopping early