I've got a 14 Sierra SLT 5.3, had it since 16, put roughly 100k miles on it. It was certified pre owned I bought it with 38k on it. For what it's worth, here's the issues I've had.
-Oil consumption @ 70k miles(all 8 rings seized on pistons), covered under warranty and fixed with updated GM pistons/rings. Completely fixed the problem, has not used a drop since (I realize you will probably not have a warranty tho)
-Rear axle seals
-High pressure fuel pump
-AC Condenser replaced. They all get a hole in them somewhere near a weld. GM knows this and a good dealer should be able to get them to cover some of (or all, I honestly don't remember) the cost, even out of warranty.
-Mine never had a problem, but I had the GM updated transmission thermostat valve installed preemptively to prevent transmission failure. Makes the transmission run cooler, valve was $70 or so.
That's all I can think of offhand. That being said, I love the truck and would buy it again. Truck has aged well, especially the interior. GM has addressed this for 22, but my SLT plus interior is definitely nicer than my buddy's 19 SLT. Mine is mostly highway miles but does see mud occasionally and has towed on some long trips. I'd say not the hardest life but definitely not the easiest either. I Blackbear tuned mine once it was out of warranty and I probably wouldn't like the truck as much as I do if not for that, made it run night and day better. I also probably wouldn't buy another one had the oil consumption not been covered under warranty. I know not all of them do that though. I have two great GM dealerships I work with that treat me awesome and really know their stuff. Hope this info helps.