Agreed, ... Neither the mechanic, nor the car owner knows what they are playing with, ... so all seems good.
There is no venting system for over-pressurisation of cylinder. Even if the CNG cylinder is safe, there are numerous fittings down stream which can leak due to a slight scratch on ferrule. Additionally tubing/fittings aren't quality manufactured/tested to save cost.
There is a fairly basic conclusion to this, if you or your mechanic isn't capable to handle hazardous equipment, just don't.