S grade they are for sure. Mostly for sieges though, where you can find alot of ppl standing in the same spot. For random ganking, most likely not, since the opponent could just run away, and leave a slow running human fighter class behind.
Warlords are by no means good at pvp. They cannot kill quickly, kite, nuke... etc. They are a slow paced low dmg pvp class. So basically if you want to have a chance you have to out skill your opponent 1v1 at least. In mass pvp situation Warlord is fun to do dmg to multiple targets. But in my experience at least... we usually get the kill shot stolen by a true dps class. Which is always dissappointing.
1vs1/etc: in general, no.., unless you know how to play one, most ppl usually choose a class that can deal out alot of dmg easely and don't need a more advanced strategy to kill someone.