Nah I don't see a flaw of his progression in strength. He can only get to be able to do so much having eaten one devil fruit already. and coming up with new skills isn't a walk in the park it takes time and real thought. on average since the start, Luffy has been one of the toughest characters if not the the strongest. He's met with each foe head on with little to no fear. I say after the time skip he will be regarded as a great pirate lord, since the series is only like half way from ever being done. and when he does become king of the pirates it wont matter how strong he was because then he is the strongest of them all. EDIT: Its slow but Luffy is at a decent pace for getting stronger, I suppose the flaw is how slow it is.