Also if you think about it, doesn't Blackbeards fruit negate your Devil's Fruit power only if he's touching you? So when they mentioned Ace's strength without the fruit, it was probably also referring to his ability to take hits, since every time he was in Blackbeards hands, that's all he did. I'm sure there has to be quite a bit more characters that can take hits like that.

So far the yami-yami fruit seems powerful, but not as powerful as BB thinks it is. It really should be a combination of BB's already immense power in combination with the fruit that he acquired.