Invis isn't a problem, constant invis builds have been around for ages and it's not hard to fight against. It will definitely be prevalent, because Hunters are the most common PvP class and Nightstalker is the only thing that just got significantly changed of the hunter abilities, but that doesn't make it broken. Spectral probably will be a problem just because it's so strong, but imo that's part of why they should make the other Nightstalker supers lower on the tier list. Deadfall should be the same tier as Well, fastest in the game. Moebius should stay where it is, mid tier, so should Spectral at the highest tier. I think a decent number of people will make the choice that 2-3 tethers is better than 1 Spectral, so it'll feel less like every Nightstalker is an automatic Spectral which will reduce player perception of frustration.