The same goes for BB the "sucking" improves the power of his punch also.
Its like throwing an egg at a brick wall.
Adding more to the concept of all offense no defense.
Of course black beard is a force to be reckoned with. He beat ace (or at least it seems that he did). But he has a weakness. Its as simple as that. And even if he can take ace's attacks, does not by any means say that luffy or anybody can go beyond ace and defeat bb. Look at when they first fought croc or eneru. At first nobody could even scratch them. Look at when someone first fought blackbeard. He got injured and I would say by the looks of it prtty severely at the end. Of course he got up and kept fighting, hes a major character in the story. Thats not the point. Anybody can do that and anybody can likewise overcome that. Besides luffy is the main character in the story. I know its terribly cliche but in terms of physicality at least, he is going to dominate everyone sooner or later.