What can go wrong, a few things.
The first thing is alignment of the frame and what material is used. I have seen a couple conversions that are horrible and back yard hack jobs where the frames were somewhat skewed, hard to tell until you looked behind the truck and it looked like a 75 nova after a hard life. One convestion had cracks where they were bolted together, somewhat of a disaster trying to find a place to happen.
The second thing is the way the frame and the extension are attached together. I have look at a couple extension jobs in Texas that the frames were reinforced with plates and both welded and bolted with grade 3 bolts in several places.
The third thing is the alignment of the rear axle(s). I know that this is very critical to tire life, seeing I had a problem with losing shims and eating tires. Replaced the tires and had a laser alignment done. Not completely happy with the truck's ride but the tires are fine now.