I think death knights are fine when i play then i do pretty well its just a matter of pumping out damage well, you literally have to do something all the time, you cant stop mashing buttons and use your runic power for spam death coils when its maxed. Death knights also have allot of survivability, although compared to warriors Death knights are severely under powered, warriors are at the top of the PVP chain and have been there for as long as i can remember.
Going to get warrior hate now
As for PVE i cant really say, i haven't raided in a very long time but i remember dual wield frost was good(that could not be the case now).