even Harpies seem afraid to go up, it because of Cliffracers, it hard to run away as they "multiplied" gathering. no wonder why harpies had to scream so loud and so far away as hagmothers can hear it.
Same as harpies can't fly up cliffs, they are rooted to the ground and just appear to hover but can't fly over unwalkable terrain.
Tell that to the harpies in blackheart haven
Yeah I'll give you that one but its just a trigger animation when they activate they fly down to attack, once they are down they behave like all the other harpies and can't fly back up.
Mounts also don't swim. Like for instance, polar bear mount, which should be VERY adept at swimming a short distance across a pond. even horses can tread water.
It's gotten better though. You can see the mount submerged somewhat in vandefall also the pets ski. Across shallow water