As for Luffy's drawback I believe that to be the happiness. Nika is always smiling, laughing and joking around which makes Luffy lax. This is going to bring the issue of someone getting hurt when Luffy's playing around with his opponent and Luffy will have to face that.
Thats what im thinking
The more Sanji and Luffy engage in tough fights, the more they will start losing their minds. Luffy becomes "the Mask" the more he fights, and Sanji might start acting weird, basically changing his whole personality.
There has to to be at least a reason for Oda naming Sanji's awakening chapter as Warrior of Science, and Luffy's as Warrior of Liberation... That might be the reason, or not, but i still think that what i said earlier about Sanji's potential CoC awakening is one of the most probable ones of happening, despite of what route Oda chooses for Luffy