I'm not sure which of my points you don't quite agree with
What I meant about cars is that servicing is not modification, if done correctly, so if you're able to get it back together at all, your not very likely to get it wrong without being aware something isn't right. (In fairness, not everyone is meticulous, which is why I say this isn't totally true).
Gas you are constrained by manufacturer's requirements, but you have to actually read the manual. Plus a lot won't be in the manual such as how to fabricate and design a gas run correctly, how to seal a flue, how to work on a gas run safely, test for leaks etc. So while an untrained but manually skilled person could arguably hang the boiler correctly if the MIs are detailed enough, the gas run itself and the act of working with gas is more dangerous than working on a car.
After all, if I balance the car on a milk crate instead of an axle stand and I'm the only one working near it, it won't injure anyone else but me.
I suppose all I really mean is that (if I understand it correctly) I could quite legally notify building control and install a boiler. Perhaps I couldn't legally do it for gain though? And that, so long as I'm conscientious, I'm less likely to create a hazard to others working on a car, or when that car is later driven, through ignorance, than working on a gas installation.