I am going to address this from a dependability standpoint as well as my own experience. I drive an 08 6.7 cummins with only 68,000 miles on it. It had a rear main seal leak at 43,000 miles and egr issues at about 46,000. I had the seal fixed and the egr and dpf deleted and haven't had any issues since.
Now, I'll share my experience with the GM trucks. You and I have messaged before and we are in the same business. We have had 5 duramax trucks in 6 years. The guy we share our warehouse with has had about a dozen in the same amount of years. The highest mileage truck in the pack is at 285,000 and the only thing replaced so far are the tires, brakes, and an alternator. That's a pre-emissions truck. 3 of the others were all dpf only trucks that we ran up to 150,000 and 180,000 miles. Again, only tires and brakes. The newer trucks with the DEF systems on them have been run from brand new to about 140,000 (I think is the highest mileage truck in the group of newer ones we have). The sensors on the DEF tank and system have been an issue on all of them. Not a huge deal but and inconvenience.
Keep in mind all of these trucks have lived with a gooseneck trailer behind them or a 1,000 gallon fertilizer tank. We haul 5, 250 gallon totes frequently. That's max towing capacity between the truck and trailer. No transmission or motor issues with any of these trucks with that kind of load almost daily. Hell the truck I drive is a 2012 duramax with 108,000 miles and still on the original brakes.