A lot of sense for what - drive-by’s?
Overly complex, over-sized, heavy - and use extended pistol magazines. Better, more versatile option is to have the pistol with variety of magazines.
If the idea is to have a submachine gun, then get the submachine gun, not an AR platform wannabe.
Some people do like them for truck guns. The ballistics of a 9mm from a 10.5 inch barrel are an improvement over pistols.