Yeah it would be so simple to, because horses already send you flying when they fall to far and die, but you eventually get back on your feet. If enemies could snipe you off your horse that would be super cool.
well not to mention the fact that horses are/do have attack moves/animations. I have no clue why the devs thought it would be good just to make them solo tag along aggro catchers, that get in the way...
/edit
when you dismount. It's like, YO, HORSE...I DISMOUNTED TO GET INTO COMBAT. W..T..F.. ARE. YOU... DOING?
Mounted combat is hard to balance. If there aren't many (or any) mounted opponents then killing people becomes easy, you can just repeatedly strafe them. If enemies can be mounted this suddenly makes combat much more difficult for unmounted individuals.
I don't use mounts, and the lack of mounted combat is a major reason for this. But I do understand the difficulty in properly implementing it.
No harder than adding a melee system.
horse has health and stamina. can perform limited amount of attacks, cooldown based. quite simple actually.
also strafing with the horses in this game would probably be harder than actually making a melee system for the horses. At least on the PC, using KB+M.
My horse experience was far from nimble and agile.