How would “Send a heap of bullets fast” be less dangerous than “send less bullets”? When a bullet gets out of the gun it has a chance to hurt someone (including innocent people). A bullet that remains in the gun hurts no-one…
If you go full auto, I suspect the shooter would find their muzzle going higher and higher so after the first couple shots they are shooting way high. So if they have 15 bullets in the magazine 3 are good and 12 are way off target type deal. Whereas if they were pulling the trigger at each shot they might aim the '12 air shots' more effectively.
Pure speculation but as I said has the feel of one of those things that may be counter intuitive to what you would initially expect.
It’s possible to fire full auto with reasonable accuracy and no brace. The amount of training it would take, though, that shooter is likely to be just as effective firing semi automatic.