@Delu Repeating crossbow are like early shotguns. They shoot many simultaneously, and just reload all of them simultaneously by just pulling a lever. It's not designed for accuracy. It's more for shooting a lot of arrows at once that no one can dodge them.
The limitation is that it's purely good for killing people. It's useless for walls.
@tlst9999 That's not the case - https://en.wikipedia.org/wiki/Repeating_crossbow#Utility most variants shoot one at a time, although some do 2 or 3. The advantage is that they do shoot quickly - just not quite as quickly as depicted here.