i wouldn't dream of screwing up my factory guns (colt 6920, plus on order I have the new KAC SR15 and CD M4LE)
while I don't see anything wrong with it, I also don't mix and match any of my franken guns (black gun, blue gun, green gun, brown gun) but that's because I keep meticulous records of shots fired, malfunctions, etc.
swapping uppers and lowers around would turn those records into garbage. besides, several of them are set up in matched ways where it's impractical or illegal to swap them.
e.g. 7" and 12" uppers only go on the registered lower, and JP LMOS upper only goes on lower with matching speed hammer, and doing something like a 12" upper on a lower that has a magpul PRS is just... goofy

