We had over 120k miles on our 2012 Yukon XL when we replaced it two years ago with the 2017 Suburban. No mechanical issues with the Yukon XL when we replaced it. The Suburban now has over 70k miles on it and again no mechanical issues.
When the AFM first came out there might have been some issues, but if you stay up on the maintenance there shouldn’t be any problems today. Other than a slightly different exhaust note the change from 8 to 4 cylinder and back is nearly unnoticeable in our Suburban.
I am not towing a large travel trailer but I would take my 3.08 equipped grocery getter over the towing 3.42’s any day unless I was going to be towing at the limits. On a recent non towing trip on two lane roads at 60mph I was pushing 29mpg. Memorial Day weekend with eight of us and a roof top carrier I was getting over 22mpg on the highway. I never saw those numbers with my 3.42 equipped Yukon XL. The Suburban also gets 2mpg better towing than the Yukon XL did.