Think about the forces involved. When used as designed, the weight on the pin box is distributed across a large flat pin box plate sitting on a large flat hitch plate centered around the king pin. When the adapter is installed, the weight is now distributed across a small adapter, significantly increasing the pounds per square inch weight factor, centered several inches offset from the king pin, and centered more toward the middle of the pin box plate rather than evenly distributed across the entire flat surface. Only surprise is that there have not been more failures.
Things are designed for a specific purpose, not for the possibility that some unknown after market manufacturer may someday make some as yet unknown adapter, of some unknown design, that will alter the stress characteristics in some as yet indefinable way. It just don't work like that.
