I don't think cav is as bad as people are stating in this thread, but there are definitely some issues with heavy cav.
For one, I think Cav is much too maneuverable, especially in more enclosed maps. If a Cav runs full speed into a wall and stops, no problem W+W and they're on their merry way in a second or two. This makes it very difficult to punish Cav who make mistakes or poor decisions when riding their horse. Yes, Cav should have a mobility advantage, they're on a horse. However, if they make a mistake it should leave them open to be punished for that mistake. If a Cav is stopped by a spear and rears or makes a mistake when charging at enemies, those are punishable just fine. But from what I see in Skirmish, Cav are riding in to situations that would be ridiculous to ride into without expecting to sacrifice themselves or at least losing their horse for the effort.
Second is that knockdowns are a little too long, I don't see a difference if a horse knocks you to the ground or a Cav rider being thrown from their horse full gallop, their horse dies and they take a tumble. Yet a player knocked down by the horse is down for quite a few seconds longer and is unable to defend themselves. While the Cav rider takes a tumble, is able to throw up his shield while on the ground allowing him the chance to get up, and is up considerably faster than the player knocked down by the horse. It needs to be tweaked so players knocked over by Cav have the same chances Cav players have when losing their horse I think.
Third, I think either there needs to be some gold adjustments to the class itself as there really isn't any option to choose lighter cav unless you really want to save the gold. They are in every way inferior to Heavy Cav, their horses are brought down faster, the rider has less armor and on a few factions lacks a shield to defend against ranged attacks, has less armor for the rider, and their less armored horse does not bump enemies as easily as an armored horse does.
Also, fall damage to horses needs to be increased on Cav, for the Desert Market map, I've seen horses choose to leap from the roof where A point is to get away as they know the horse will survive the fall rather than try to get themselves out of a bad situation in a more reasonable fashion. Likely could make it so the rider also receives a portion of fall damage rather than just the horse, but I think it should be increased a bit further regardless.
I think making some changes in those areas and opinions on Cav being op might change quite a bit.