I used to be an automotive diesel engine mechanic as well at Diesel Injection Sales and Service, San Antonio TX. Ive seen my fair share of cummins wrist pins letting go and dont get me started on powerstrokes. you know how the injector harness incorporates the valve cover gasket and fries up killing a whole bank of injectors. powerstroke injectors arent cheap either. Injection controller modules. then theres the o rings on the injectors cracking and filling the whole fuel tank with engine oil.
The cummins is probably the best of the three, but the powerstroke is more similar to a boat anchor than a duramax.
OK, you don't like powerstrokes. That does not mean Duramaxes are not crap. I do a good amount of work on International 444e motors, which are almost identical to the power strokes. Yeah, they have issues all their own. But IME, they have about twice the longevity of the Duramaxes and $10k will get you a brand new reman installed and running. I've salvaged quite a few with just a hone, some rings, pass-thrus, and a set of rockers. That never seems to be an option with the Duramaxes--they're toast.
A duramax injector costs around $50-100 more than a PS/International injector, depending on where you shop. Reman, anyway. New is closer to a $300 difference.
I have seen Duramax return lines crack and fill the crankcases with fuel, destroying the engine by the time it's caught, more often than I have seen HEUI injector o-rings give out and dump oil into the fuel tanks. Usually people notice that one when the engine shuts down from lack of actuator pressure long before the engine gets damaged by low oil.
Duramaxes have an expected longevity (from GM) of 100k, which is fairly accurate (the best seem to be close to 120-150k) and an absolute joke in a diesel. Plus, they have more failures which require pulling the engine out, like the water passage through the bell housing springing a leak. Between the high failure rates, high parts prices, low lifetimes, and ridiculous labor costs you couldn't give me one of the stupid things--unless I could sell it.
Of course, my experience is in the world of commercial trucks, where the vehicles are actually loaded down. In pickups that simply have lots of power and nothing to haul but a bedliner and maybe a cooler, yeah, I could imagine a Duramax being a little more dependable.