R134 refrigerant is good, but does not want to reject heat quick enough, hence it needs to be channeled in very thin tubes to reject the superheat gained from being compressed. Similarly it also needs a very thin tube evaporator to heat up (boil) as it does not like to accept heat like the old R12 did.
R12 on the other hand was discontinued mostly because DuPont's patent expired. It was used in older A/Cs which were no doubt heavy duty but also had thicker designs making the heat transfer slower (the gas took up for it)
If you are trying to run R134 in a system with a small serpentine condenser and thick evaporator - forget about cooling, pray it barely runs. Either you should have a very large condenser with multiple high speed fans or a very efficient evaporator with slow moving air.
@ cracked
If your A/C serviceman told you that statement of low pressure gas etc and is not used for cars - remove your car from his shop as soon as possible, the person is not qualified to service a car's A/C
