http://www.onemanga.com/One_Piece/441/02/
1. He seems to be very stationary during the fight, which is a very bad thing due to his weakness, he's a sitting duck
2. His vortex is a very very risky trick and wont really work against people his size. It worked well with ace cause ace is alot smaller so he was able to grab ace as if he were a child.
3. When his DF is activated the darkness only runs over his back which is a very bad thing singe he cant phase through attacks and he doesnt move either. He's also a huge target. There are alot of people out there who can move faster than BB can activate his vortex.
He can improve in all those areas. In my eyes BB isnt one of the strongest people in this manga since he has some major weaknesses.
Put him against someone who uses Weapons or Rankyaku and he'd be a dead man. Zoro would split his stomach open.
Notice that Ace landed quite a few attacks on him. Had that been one of zoro's flying slashes or Kaku's Rankyaku he would've been dead.
His power is very great but he's a massive sitting duck and given the fact that he cant cover his entire body with the vortex thats a MAJOR weakness.
Liberation is very cool but it would be alot better if he could utilize Liberation in his palm like Vortex
He has alot of areas to improve on.
People dont get that He'll be the most powerful villain EVENTUALLY, but he's not right now. Like luffy he has yet to master his power. That will happen gradually.
In doing that Oda just proves his genius. A villain who actually has to grow his power over the course of the manga not God-like from the get-go