It’s all about preventing 80% of the problems for 20% of the cost. But I think you will prevent 90% of the problems paying a mechanic to look at the vehicle on the lot and then, if you win, take it to be inspected by professionals.
I agree if you want the best guarantee, then buying it from a dealer is best.
If the inspections included a scoping of the engine, transmission, & the rear end differential, perhaps, but of course they don't do that.
Best to just invest in the extended warranty.