The great irony is it's frequently the "ductwork" that's the problem: plugged or badly installed exhaust pipes, which the manufacture has no control over. The rest are the appliance itself wearing out or failing with no warning.
I've repaired furnaces myself several times including replacing burners and exhaust fans -- it isn't rocket science. It's no different than working on any other "dangerous" thing like a car. If someone somehow manages to fuck up so badly it hurts or kills someone that's on them.