Are handgun compensator's legal in Nebraska?
Sure.

Plenty of people use them---especially if they are shooting Open division in USPSA. (And there are factory-compensated versions of a number of common guns available also.)
There are lots of cheap drop-in threaded barrels for various common guns where you can also find cheap comps for them, too.
Now, if you had asked whether or not those comps are worthwhile, that's a different question.....

(In an Open gun, they certainly are. In a carry gun.....not so much, IMO.)