For me, it's all about the risk/reward tradeoff. First, let me say that I don't carry anything on the back of my 5th-wheel at the present time, but since a welder has inspected my rear bumper and declared it safe to carry up to 2 bikes, I would even though there are arguments against it on this forum.
My decision is based on the likelihood of them falling off. I am convinced that bikes have fallen off the back of bumper mounted hitches, but the lack of evidence to show that this is a common occurrence supports my decision to accept the risk and enjoy the reward.
Is it possible for a failure to happen to me? Yes.
What are the odds that a failure will occur with my specific bumper construction? Extremely low.
It's like the decision we make every time we get on the road.
Could we be in an accident on a given trip? Yes
Is it likely we will? No