The haitian's powers are wierd. We obviously know he can extract memories, but the whole power-nullifying effect is iffy. He cancelled out Matt's and Eden's power, and i'm not sure if it was him or something else that stopped Sylar's power when the Company had him.
On the other hand, he wasn't able to stop Nathan from flying away, and it seems odd that in the end of Company Man, after taking the Bennet family to safety he didn't go back in to stop Ted going nuclear. If he could cancel out any power, he would've done something.
Someone suggested he can only cancel out mental powers, which makes sense. It works on Matt and Eden, but not on Ted and Nathan. Who knows, though. The Haitian's just a huge mystery.