You will see problems at about 80,000 miles when you need a new DPF. Not cheap. This will erase all fuel mileage savings over the 5.3.
We have 3.0's well into the 100,00's, never had one go bad, never replaced one, but I am sure there are a few out there that failed. DPF's can be cleaned, which is done on Class 8 trucks at 500,000, I haven't seen any GM documentation on when, or if it needs to be removed and sent out for cleaning. I had my in-house mechanic look, we can buy an OEM DPF from GM for a little over $900 our cost, not sure what retail is, but the Class 8 trucks I know what they cost to clean, around $1,500, and that is just the cleaning, doesn't include R&R from the truck. I wish GM would have included a manual regen, and a way to tell you it's needs one, or if it is doing a regen, but I have a feeling some folks would panic if it told.