You said you're skeptical about people who have 18,000 hours who fly general aviation, or who gained that experience in general aviation.
It's your constant reference to these hourly totals, which (except in highly unusual - in Europe - occupations) cannot have any relevance to this
piston GA engine reliability debate.
what is the likely hood of an engine failure in GA?
I think the best order of magnitude guide is what was posted earlier by Englishal. Unless you are running particularly dubious practices, you are unlikely to get one before you finally fail your medical...
Unfortunately some people choose to interpret such a statement as saying that one should not train for one, etc. which is obviously completely wrong.