Invisibility is difficult, but most systems that try to address it handle the situations that
1. Invisibility ends when you try to attack - obviously you are "less invisible" when you are hitting someone :P
2. Invisibility doesn't work so well in the heat of battle - i.e "going stealth" shouldn't really be possible, or should have a very high failure rate if you are in the middle of enemies.

On that note, we should remember that the Character Class is not Ninja, but Assassin. An assassin is a silent killer, who kills after preparation. A ninja is a warrior with specialised skills for death (not tanking, or brawling, or taking on 5 people at a time, just killing one on one perhaps).

Anyway, I think the opportunity to have a balanced class with some kind of stealth option would be very cool.