I think the biggest thing here is that it's so unique that it's almost too much. The nice thing about the game right now is that every light gun can go on every ship, and every heavy gun can go on any ship that can carry heavy guns. So if I don't like brawling, I can outfit my ship to be a sniper. If I love to pop balloons and destroy guns, I can do that, and I can do that on whatever ship I want. Now, not every strategy works for every ship, but it's still my choice.
This gun would change that- you'd always have to play this ship in one manner, where you get above the enemy and shoot down at them. And if they ever got above you, you'd lose access to your one claim to fame ... and that's not even mentioning how much a Blenderfish would ruin your day.
I think the concept of ball turrets are cool, and in a more three dimensional air combat game could work really well (I'd love to see a WWII-era fighter plane simulator that has you in a flying fortress), but Guns of Icarus is still too much about the horizontal. Vertical positioning is important, but the focus is still on forward/back and left/right- this gun wrecks the curve, and not in a way that screams "easily balanced" to me.