Not all of these are hydrocarbons, some of the more interesting alternatives may be worth taking a look at.
The big problem with R134a - other than the fact that it doesn't cool as well as R12 - is it's not compatible with the mineral oil used to lubricate the compressor in R12 systems. By compatible I mean it is not miscible - it does not carry the oil through the system and return it to the compressor. Just filling an R12 system with R134a will cause the compressor to die in short order.
Hence along came some of these blends. Many are comprised of 80% or more R134a, with another component added in an attempt to move the mineral oil through the system. I believe Freeze-12 falls into this category, among others.
I'm personally running Autofrost - R406a - in my 124 wagon. I don't have enough time on it yet to say it won't break with any authority, but it sure does cool great. On the highway I'm seeing a 37 degree drop from ambient, which is very good. It's almost too darn cold, which is not a complaint one hears very often about the 124 series. Anyways, R406a merits a 'nonflammable' rating. It's a blend of three components: R-22, R142b, & 4% isobutane. The latter helps it move the refrigerant oil through the system. So I have ~1fl ounce of isobutane in my a/c system. Consdering it is also filled with 8 ounces of lubricating oil, I'm not gonna worry about it too much. The cost is ~$9/can, so it's a lot cheaper than R-12, though actually more expensive than R-134a.
Given all the glass area of a wagon, and that I live in Texas, I'd sell the wagon before converting it to R134a. It just doesn't work well enough.