Horse crossbowmen existed in the time period, saying it's unrealistic is funny. Yes, the reloading is done differently (hint: you use your foot), and you could definitely not maneuver like you can in the game, but horse maneuver in crpg (at speed, at any rate) is heh anyway.
The class was a bazillion times more fun to play when the crossbows were less accurate and more damaging, way back when. And one point of HA skill was 12 agi, if I remember correctly. So you had an inaccurate shotgun with 2 HA at most. But it'd kill a lancer in the chest, however, if you could hit.