Propane actually cools quite well and you will hear success stories from people who use it. I don't consider it to be any more dangerous than having pressurized gasoline under your hood but many states have outlawed propane as a refrigerant.

One very real problem with using propane that you will not hear when reading the success stories is that propane is too light to circulate the lubricant in your AC system. This could lead to lubricant related failures. The successful propane conversions used a second heavier refrigerant to move the lubricant. Then you get into the same old problems with fractionalization of blends and you are better off to avoid them.