I have seen (personally, not in pictures) at least four trucks that had cracks in the chassis as a direct result of a hard mounted body.
Obviously, hard mounting is a tried and tested method on tens of thousands of goods trucks, no one can argue that. I guess that it comes down to what you do with the truck. If it spends most of its life on the black top then it may go through its life with no frame issues, but use it off road with a heavy camper that is twisting the chassis continually and focusing that torsional stress between the cab and the step and you increase the potential for failure of the chassis.
If you have a hard mounted camper on a FG I do not think that anyone would dispute that it will cause more concentrated stress to the chassis when taken off road than if the camper were to be spring mounted.
Is a hard mounted camper guaranteed to have a frame failure? Absolutely not, but the possibility of this happening is significantly increased.