The problem with g5 is too much loony toons in serious fights, and the mega retcon needed to make g5 happen.
But the animation looks very good at least.
Pretty much my biggest issue with it too. People defend it by saying Luffy's fights were always 'loony toons' but Luffy played his fights straight with occasional comedic moments, which was true for almost every fight, not just Luffy's. Like during Arlong Park, you have Luffy swatting away fishmen and getting ready to fight, only to realize his feet are stuck in concrete, then Hatchan doing something silly while fighting Zoro who stays serious, but then once Luffy is saved by Nojiko, the fight is pure drama and action.
When you get rid of the latter and make it pure comedy with Luffy showing zero tension, it takes away from what made OP special in how it could turn something silly like rubber limbs and make them feel like a natural part of a life or death battle and how they could realistically be used in a fight.