Yeah they dont pack the punch of a european one. As far as I know the heavier single shot crossbows could fire like 300 yards (not accurate at that range though), while the repeating one was only like 50-75 yards according to that page (again im not sure if its totally accurate hehe). Plus you actually took time to aim the single shot crossbows, the repeating one you just point in general direction and loose bolts like a madman lol. The pound force of the bow on it obviously couldnt be nearly that of the single shot crossbow, since it would have to be able to be drawn back and loosed without bracing it against something, and ofcourse easy enough to be able to fire that rapidly.
Different tactics really. The single shot crossbows would fire slowly but more accurate and farther, while the repeating crossbows would fire at an incredible rate but wouldnt have the range or the accuracy.
This could be simulated in M&B by making the repeating crossbow have a lower accuracy, higher firing speed, lower velocity, quantity of 10, and reload like a pistol (the real one could easily be reloaded on the run im sure, I mean all you do is drop more bolts in the top).