whats wrong with their lowers?
Other than the hour or so needed to deburr everything and clearance the lower to fit your upper - their lowers hold a mil-spec upper off-center at the rear, because of where they positioned the recess for the rear lug. I sent mine back twice, the first time the replacement had the same issue; it's still back with them again, on my dime.
Like this; it rubs the bcg against the buffer tube: