The Nossferatu are a very interesting class to play. To answer a few of your questions:
There are a few quests that are difficult to complete, but as far as I know, you can finish them all.
One example is the Thin Blood quest in the beginning of the game. You are pointed towards the Diner in order to find more info about the girl, but the old woman won't talk to you (for a little fun, keep on talking to her and you'll be able to kill her with a heart attack)! Just check behind her to pick up the purse.
Obfuscate is my favorite discipline, and the Nossferatu are a very good clan with which to use it. There are many quests in which you must not be seen, or can receive more expirience for doing it stealthily. In these cases, the obfuscate power really shines.
Another way in which obfuscate is useful is the ability to easily stealth kill almost anyone in the game (some notable exeptions like bosses cannot be stealth killed) and the huge damage bonus you receive when hitting someone while in obfuscate (triple damage at top level).
All in all, play a Nossferatu if you like things done quickly and efficiently, but be aware that many people will simply shoot you on sight. However, don't think that you cannot be persuasive as a Nossferatu. I had a persuade of seven at the end of the game, and though it simply won't work with some people, it does with others.
