Propane has long been used as a refrigerant, and was used long before r12 w/o issues. There also isn't that much actual propane in the cans, and IIRC I only used two of the cans to fill the system. If it was to leak out during an impact it would be so quick it wouldn't be an issue, and would/should be able to detect it if it leaked into the cab, whereas there isn't a smell w/R134.
The report I read about testicle tumors was in a study done w/rats, so the levels used may be more determinant than simple exposure, but it is hazardous and can be deadly if you inhale the gas.
Actually IIRC R134 is also being discontinued since it also contains ozone depleting properties.
I didn't know that r-134a was being discontinued
