His failure/fire was caused by human error, not that of the hydrocarbon based refrigerant.  You have a higher chance of engine fire being caused from leaking injectors/carb, or oil onto the exhaust manifold than from the amount of gas in the refrigerant in the A/C system.

I will take my chances w/the slim chance of fire, over the higher chance of testicular cancer from a leaking R134 system. 

Hydrocarbons have long been used as a refrigerant, if the risk of fire was so high that it was dangerous to use it, it would not still be in use.