Never saw any trouble here in Texas, my view would be possibly in Arizona? If car vapor locks the pump output is down, blocking off bypass increases pump pressure a bit, which helps stop vapor lock. Both bypass and non-bypass pumps used same size diaphragm, only change was the bypass. Another whole circuit to leak in my view.
Never had a customer complain either.
Like they say, use as much metal line as possible and never use rubber in the lowest run, only on the up or down legs and short as possible there.