Are you actually referring to automotive propane system that meet NFPA 58? Even if the fuel line fails in an automotive system, the excess flow valve will prevent an uncontrolled release of propane. In a vehicle fire, the safety relief valve is designed to prevent explosions with controlled blow-offs. Never put fuel into a propane conversion until it has been inspected and complies with the LPG installation code in your jurisdiction.

Since propane only has 74% of the energy of a gallon of gasoline, it will obviously get lower fuel economy but a propane vehicle typically gets better fuel economy than the energy comparison would predict. Old-style mixer systems typically get 75-80% of a vehicle's fuel economy. Modern injection conversions easily get over 80%. If you are buying propane for less than 74% of the price of gasoline (easily done), your cost per mile (or km) will be lower.

YouTube - LPG Car Fire Tests