Well, in my dirt poor past, I would overhaul 4 strokes by doing a valve job, piston and rings. However I always overlooked some worn out component or other. I would leave in: worn cam chains, worn cam bearing races in the head, recut old valves, worn valve guides and of course the crankshaft. (like I said, a dirt poor overhaul).
This did result in less than preditable reliability. A fact which could have easily been predicted:)
SO, I believe that engine life is predictable with new parts, whether 2 or 4 stroke. An hourmeter and a compression gauge will help provide the method to achieve the predictability. For instance, 25 hours and compression drops by 3%. 50 hours and compression drops by 10%. 45 hours might be a good time to renew the rings/cylinder/piston/valves. A careful measurement of worn parts after disassembly will help the evaluation.
Chris