There are similar myths against the .50BMG as an anti-personnel weapon, some believe it is anti-materiel only, and illegal for other purposes. "You have to shoot at the guy's backpack / belt buckle / eyeglasses / etc" for the shot to be legal.
There is a good article from the Marine Corps Association that says otherwise:
.50BMG is OK for killing in war
The problem with a .50 is weight and logistics. 30 rounds of 5.56 weight-wise is minuscule compared to the same of BMG. The rifle is longer and weighs up to 35 lbs.
And the smoke cloud (from smokeLESS powder) and noise are pretty good pinpoints.
But if the target is a mile+ away......