I don't know if I'd put all that on his ability, to be honest. You're referring to the fight with him and Kid on Sabaody? There could be any number of reasons why their fight wasn't shown; namely Oda not wanting to have to show too much of the characters(including Law's) fighting abilities. I'm pretty sure he gave his heart to Caesar and got Monet's in exchange, as a part of a guarantee of them working together, again nothing to really do with his ability. His ability doesn't make him bulletproof now does it? And I think we can agree that DD is stronger. Also about his ability, just because his "ROOM" allows him to manipulate his targets within that range, it doesn't make them sitting ducks, negate their devil fruit or other skills that they have. So if he comes across a fighter of similar level or stronger, he can't just rely on that. Personally I'm actually hoping to see more of his swordsmanship in combination with his df, I want to see how proficient of a swordsman he actually is.