Why would one piece buckets be any worse in a FiST vs other cars?
Supposedly because they are NOT designed to be able to fail/give way backwards (towards the rear of the car), like a recliner is, and if/when they do, they crack with sharp shards which will then be driven into your spine (among other reasons I hope Rick, and others, will come on and explain).
Also, it IS a hazard in every other car, but the owners just don't care, or are totally oblivious to the dangers involved.
(UNLESS the car was actually designed/crash tested for shells, and usually it is in cars with some sort of bulkhead right behind the driver, as in a roadster, or mid-engined car.)
I wanted to do light racing shells as well, but they convinced me otherwise, in a daily driven, mostly street car.
Besides, you will eventually crack/deform the outside bolster of the shell from ingress/egress, on a daily driver, no matter how careful you are to lift yourself up and over that bolster every time.