I'd agree that the pro4 doesn't have much "give" in the drivetrain.
It's not too much of a stretch of your imagination to see that suddenly locking a wheel in a crash will result in a break or bend somewhere.
The TiR units i'm running have survived extremely well. I had a friend drive my car and he managed to drill it into a wooden board (practice day - so no corner dots) and the car survived unscathed save for a wheel bearing and a few loose screws.
Shaft driven cars in particular aren't as forgiving as a belt. My old schumacher NEVER broke a cvd, and those were the plastic units! (I was however, running a front diff also)