Rear sight can't be adjusted for elevation.
C'est la vie.
A lot of rear BUIS brands have windage adjustment only.
Both sights won't lock unfolded.
Wut? I don't get it.
I think you are referring to the fact
that you HAVE to press the lock to deploy,
but in order to fold (flip) it back you don't have to touch the lock.
If that is the case, that feature was INTENDED to be that way.
Cannot be mounted on a gas block.
Here you go. Finally, somebody said it.
You mount the Magpul on a gas block and it will melt like mozarella
(because it's plashtik).
However, you won't find that many FSB's with the picatinny deck
at the same level with the flat top receiver.
Usually the FSB's have the picatinny at a lower level,
therefore, the magpul front BUIS will have to be placed
on the top rail of the handguard.