I would not buy any 1/2 ton by any of these manufacturers no matter what engine it has.

I have a deposit down on that exact truck.  I’m going in with no illusions. It is not a perfect truck, but it is the best for what I want.

pros

Smooth, quiet, good mileage.  It comes only with the 10-speed, which has long legs and avoids the problems of the GM 8-speed (8L90?).

cons

The 0w-20 Dexos D oil is hard to find; I think right now only Mobil and AC Delco produce the spec oil (and don’t even think about substituting something “close”).  Crank-no-start is still a mystery (look it up and understand this issue fully).  Oil pump belt thing (look that up too).  Not a brand new motor any more, but there aren’t too many units that have gone over 100,000 miles so long term is still a “?”.

As far as the rest of the truck, there are + and - there too.  IMO, the Sierra AT4 is the best looking truck on the market right now (with the Ram 1500). The interior is kinda blah and the seats are hard, but it’s functional.  And you can assume electrical gremlins from time to time.

It’s a really high tech engine (look up the variable cooling system, for example), and it already has a reputation for being fussy about maintenance.  It ain’t an old Ford 7.3 that will run forever on bacon grease and coffee grounds.  GMC pays for the first oil change.  I plan to get that at 5000.  After that, I’ll do my own maintenance: oil+filter every 5000, air and fuel filters every 10,000 (if not every 5000). Gonna use a fuel additive from day 1, too.

There’s no perfect truck.  Even the new Tundra has its flaws.  But if he keeps ahead on maintenance and uses good quality fuel, it should be reliable enough.

PS: don’t use the block heater above 0 F.

https://carkiller.com/scottykilmer/?wpfs=duramax

Well the motor seems decent, however,  GM is so far down the rabbit hole with quality control I wouldn’t even look in their direction. 

The engine is fine, but the rest of the vehicle (especially electronics) will drag down the overall quality/reliability.  Also, the 10-speed automatic transmission is not proven so jury is out on its longevity.  If he is dead set on getting it, my recommendation is to go light on the options/vehicle packages so there are fewer things to break.

For an unbiased opinion, take a look at Tim Esterdahl from Pickup Truck Talk. He has a few videos on his channel talking about the Sierra AT4 Duramax 

https://duckduckgo.com/?q=pickup+truck+talk+at4&t=iphone&iar=videos&iax=videos&ia=videos&pn=1&iai=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3Ds1cXh2oeW3M