The picture posted of the small broken frame looks like any low cost 5th wheel trailer frame that didn't have the proper size material or gusseting for the job. WE see this type/similar breakage even with a reg 5th wheel hitch such as problems IB516 had on his larger trailer.
A broken/cracked pin box and surrounding framing cracks where its attached is where problems can occur if it wasn't designed and built for a GN adapter use.
If the trailer in question has been towed with a adapter and you want to use it I would watch close when accelerating or hard breaking if there is any movement around the pin box attach area. Use your own judgement.