I was wondering if it didnt need some balance factor, hmm wonder if while using it you forgo one of your extra attacks to be prepared defensively for the swarms you face.
I only saw it used once in an actual game. GM did not know 5e. Gradually just let anything go, so casually he moved a half dozen goblins at once into position to melee the front line against a glaive-sentinel-tunnel-fighter and then started to make attack when said UA vegematic stopped him for the slaughter.
It was not too long after that I think that the GM gave up and the game folded after another round of griping about 5e for half a session.