You can't generate nearly as much force on a fg with say 48/16 gearing as you can with 22/32 on a MTB and qr are used on those bikes all the time.
Yea, but most MTB use horizontal dropouts, so most of that force is on the actualy dropout, not the QR. The QR really just needs to be tight enough to the wheel doesn't fall out. Of course disc brakes cause a whoe mess of issues.