[Freon]

You don't hear many stories of people breaking either transmission, so it would seem the answer is, "more than anyone has bothered to put in their 135i."

A lot of this has to do with drivetrain slop (dual mass flywheel, driveshaft isolator, bushings, etc), tires, and abuse. I wouldn't worry too much until you are running on slicks with a solid flywheel, unsprung clutch, and beating the hell out of it.