Well, my interest was piqued on HC-12a/ES-12a (Propane and IsoButane) so I did a few hours of research.
ES-12a is nearly a perfect drop in replacement for R-12. No need to replace anything. Compatible with just about any oil, properties similar to R-12, no need to replace any hoses or other gear, if it escapes it has little if any effect on ozone layer or global warming, degrades rapidly so it doesn't build up in the environment. Oh, and it cools efficiently while drawing less energy from the host system.
However, it is flammable if the gas escapes into an oxygen environment. Very high flashpoint - higher than R-134a it is said. (R-134a degrades to poisonous gasses in the presence of temperatures not much higher that a burning cigarette).
Seems kinda stupid for the EPA to make ES-12a illegal when it's perfectly legal to drive an oversize gas can in the first place. It's generally ok to buy it but you can't use it in any system that has used R-12. Also, you may not vent the gas but gas leaks out of cooking systems all the time and from gas lines, storage tanks and delivery trucks.
Furthmore, it's ok to use R-134a which still depletes the ozone layer just not as much as R-12/22. It's also ok to use blends of R-22 of less than or equal to 50% along with other ozone depleting HCFC components.
Who makes these laws?
Lobbyists?