R134a is known to be 80% as efficient as R12, so in hotter climates it is a factor. I have a 1980 300D that I converted to R134a. I replaced the compressor, TXV, receiver/dryer, and refigerant. The 123 a/c systems were not robust to start so the 20% loss in efficiency makes a big difference. Also, Freeze12 costs more than R134a, not R12. It is still more economical than R12 and easier to install than R134a.
While I know most pros don't want to touch anything other than R12 and R134a for fear of contaminating their recovery systems, I still think the compatibility of Freeze12 to R12 makes it a viable alternative.