Some people are mad about Scratch, but it's important to remember that the most dangerous time to fight a Fruit user is the first time, when you don't know what the hell they are or what they can do. It's probably got something to do with haki and sound waves, but Luffy doesn't know that and neither does Zoro. He's also one of the Supernovas, who are (ALLEGEDLY) all approximately equal to or around Luffy's own level, though Luffy has probably surpassed most of them given his last few recent powerups.
It's also important to remember Luffy's own weaknesses. Physical blows do almost nothing to him, he was trading punches with haki users all the way back in the Ciper Pol arc, and he's always been functionally immune to gunfire and even cannonfire as well because of it. His weaknesses are cuts and explosions, because rubber won't help with either. What was it that Scratchman stunned him with? Not the punch, which he shrugged off, or the cut, when he sent at Zoro instead. The explosion. Which makes sense. If there was ever a cheap way to beat Luffy, it would be to just chuck dynamite and fire at him endlessly while keeping your distance. We'll see if Scratchman is smart enough to do that, or if he'll just be a cocky bastard. Or maybe Kidd will deal with him, we'll see.