If the question is, can a spare tire mount or bumper fatigue or loosen up and fall off the TV. Of course it can and some do. Most do not.
I have also see a wheel come off of a moving car. That was scary because it passed in front of our vehicle.
If you worry about it do not do it. My concern with mounting a bike or two is the vibration and twisting torque while the bikes move down the road. I worry about that so I haul my bikes in the truck bed.
I also do not like the crud that builds up on the back of the TT.
I an currently contemplating a front hitch receiver.