My thoughts are this,
1 - 5.7 little high speed bullet, same draw backs as 5.56, not enough weight or mass to dump a bunch of energy into the target, tendency to punch little holes right through it . Cold blooded as it may sound, it did not seem to be particularly effective for the Ft Hood shooter.
2 - Because FN is the only manufacturer of arms in this caliber ammo could be harder to come by whether through availability or price.
3 - same round in pistol and carbine GOOD
4 - Butt load of capacity is also a very good thing
5 - Every one I've heard of that has shot one speaks very well of the handling, reliability and accuracy of both carbine and pistol.