if you want it to just charge into enemy you can do it by burning their tail or other parts basically scare them to make them panic and stampeding the people in its front (may including yours) without riding it.
but if you want to ride it as cavalry, it have horn that can get hook or tied by rope such as lasso by enemy which may can stop its momentum, it also not as fast as horse (unless in angry mode as far as i read from google, but i guess it not last long just a sudden burst) so maneuverability is poor against other cavalry type, but i believe genetic manipulation may can fix it, just my opinion.
also that horn may can end up stuck with a lot of corpse weighting/dragging or slowing it down, and to remove it, he need to swing forcefully which is not that good for cavalry i guess....... unless the person can design special type of saddle to not get thrown out, but still that action (in attempt to remove the corpse) probably hampering/slowing it movement/momentum too, including disturbing the rider action in attempt to attack its opponent, and can endanger the rider from his own weapon.
unless the rider is so skilled, strong, and fast enough to remove the stuck up corpse, after thinking again this may make the rider develop a tool for that specific reason, but i dont know what kind of tool it be. still i guess it basically to much chore compare to horse.
so you probably need to cut/remove the horn to make it viable.