The only problem with the Nebraska is it has too much Armor Penetration with HE.
From a balancing standpoint, all it needs is a Armor Penetration nerf. Everything else about the Nebraska is well balanced with the other long ranged BB6s (Kaiser, P24)
The only other major issue is all the BB6s turn too fast.
HA/Fleet vs Fleet/Competitive environment:
Offensive:
Nebraska=Kaiser>QV>Charles>Italy>Amagi=P24
Defensive:
Kaiser=Nebraska>P24>QV=Italy>Amagi>Charles
Balanced:
Nebraska>QV>Kaiser=P24>Italy>Amagi>Charles (Due to that large target on the back :P
Also tempted to put P24 in first for defense, but it's speed compared to Charlemagne sorta negatived doing that.
GB/Regular game-play
Nebraska>QV>Charlesmagne>Italy>Amagi>Kasier>P24
With Armor Penetration correction, QV would be on top over Nebraska.
Amagi and P24 near the end due to High angle, plenty of time for a fast ship to dodge if the players is not well trained and patient.
Kaiser due to its lack of a punch vs the QV is well back. Range can't save you if some guy can noob it up in a QV get into range and rick-roll you.
Also remember, the BB6s are not balanced for Great Battles, they are balanced against eachother for Fleet vs Fleet/Competitive gameplay.