you can be a NB in PvP. Just expect to be more frustrated than having fun. and I don't mean by getting killed a lot, I mean just plain out frustrated. The class has many bugs, abilities that don't work, and a NBs damage does not come anywhere near a sorcerer and a dragonknights. The class is, in my opinon, not yet ready for PvP. Just stick to PvE with it till tis fixed and PvP with one of the other 3 classes.

You can still have fun being a nightblade in pvp for sure. I am a wood elf bow nightblade with resto swap and mostly use the assasination and shadow skill lines. The biggest thing I would say about being a nightblade is you want extremely high opening damage because once out of stealth if you are a stamina build then your overall dps will be not that great. From this I would suggest a khajiit or woodelf because of the increased opening damage. If you plan to go melee tank or straight survivability then imperial has red diamond or whatever the skill is called that heals you when you do damage. What I would suggest though is get your opener to do as much damage as possible so all it takes is a few hits and a killers blade to wipe someone.

Edit: Also if you are outnumbered try out your shadow ultimate it is very good. It allows you to tank tons of people, while still doing damage. If your not melee based and use ranged like a bow, try flawless dawnbreaker to boost that damage. Otherwise if you are melee you would want a assasination ultimate morph.

2nd Edit: Normally you want a mixture of your class skills (shadow, assassination, and siphoning). You may want siphoning for getting back up to full hp and debuffs etc. You dont have to use them all though if you do not want to though. Like for me instead of using siphoning I use vampire drain and resto staff for my stat recovery. It really comes down to what you think is necessary to leave on your bar. Good luck out there.