Propane works very, very similarly to R-12 in air conditioning systems- the gas/liquid transition temperature and pressure are very close and it readily transports ordinary, old-school mineral lubricating oil. It was a leading candidate to replace R-12, but the flammability issue and industry pressure nixed it except in the quasi-illegal aftermarket realm (regulated shops are at risk if they use it, but you can do whatever you want to your own car without much fear). Now I know, its not really more flammable in a car accident than the oil-loaded mist that can spray out of an R-12 or R-134a system... but I would be a little bit worried about an evaporator leak that lets propane settle into the lowest part of the ductwork... then you start the car and turn on the blower with its arcing commutator... even that's not likely to produce a deadly explosion, but could make enough of a "fuh-WHUMP" to do some damage and injury.
And come on, seriously: Haven't R-134a systems been the most reliable AC systems any of us have ever had? My first R-134a system was in a 1993 vehicle, and it had ONE issue over the course of 250,000 miles- the original evaporator was prone to corrosion and was replaced under a recall. I've converted two old R-12 systems to R-134a (one using an original 1969 Chrysler RV2 compressor), and they've been booking along fine for 10 YEARS! I wouldn't want to go back to R-12 OR propane, necessarily, given how well 134a is working out despite all the dire fears that were rampant in the 1990s.