It would seem that using an un-ramped carrier in a 9mm AR = a broken hammer pin sooner or later. The ramp on the carrier slows down the cocking stroke of the hammer to a more reasonable speed, progressively depressing the hammer as the carrier moves rearward. Without the ramp, the hammer gets slammed back as soon as the carrier begins to move rearward, leading to the broken hammer pins.
Another aspect of this situation besides the obvious broken hammer pins is the extreme hammering that your lower receiver's hammer pin holes receive from using an unramped carrier. If the pin is getting whacked hard enough to break it, you've gotta figure that the holes are taking an extraordinary beating as well! Yet another would be the receiver damage resulting from the broken hammer pin "wallowing and gouging around" in the receiver, after it gets that fatal hit which breaks it.
If you look at any 5.56mm AR-15/M-16 bolt carrier, shrouded or unshrouded, you will notice it is ramped, and this is so for good reason. It seems like the person who designed the unramped 9mm carrier was asleep at the wheel, or didn't have a very good grasp of physics.