No. It's very well within the limits of the game. You can definitely still lose with two Nachghuls.
Also, if you do feel like you're going to table a newbie then the Nachtghul is actually a good unit to have because you can simply "forget" to deploy it.
More broadly I don't think that people should worry about this stuff when building lists. Because there's broadly only two kinds of players: Those who hold back to facilitate a more fun experience, and those who try their hardest to crush their opponent. If you're the former then I think you should hold back during the game, rather than in your list building. Because if you hold back during list building, you'll just get your ass kicked every time you play a power gamer.
The "is it narrative?" filter is a really crap way of determining this stuff for two reasons: Firstly, every player will have a different idea of what is narrative/fluffy. Secondly, some gangs are very powerful when you play them as inended (e.g. Corpse Grinders an Van Saar).