Nightsisters seem very useless to me.

I'm relatively newer to the game (sub 2 mil GP), but i bought the hyperdrive bundle and first thing i did was focus on my NS squad. I am extremely happy with them and that early in the game they are one of the best squads. they kept me solidly in top 50 in arena (with very little other money spent - at the time) and only recently slipped out of top 50 but easily top 100. They can take out equally powered JKR teams easily, bastilla lead jedi are no issue at all. I am 50-50 against Padme teams of equal strength as well. Only dropping out of top 50 because they definetly can't win against DR squads.
They are also very good in raids and territory battles, and do great in GAC.

So my experience is NS squad is actually incredible, especially for early - mid game