Come again? For starters, you can't hit a durn thing with one. Have you ever had the pleasure of firing one? Tons of fun, that's about it.
The HK91 in full auto is a whole lot to handle, but the HK MP5 isn't a problem to control with a bit of practice. Accuracy may be less than stellar, but more than sufficient for combat. Carlos Hathcock used an M2 .50 BMG for sniping.
Not all full auto guns handle the same way and a little training goes a long way to learning to use them effectively.
As for the AWB or NFA failing as indicated by the North Hollywood incident, then every law broken is a failure too? No.