It's just the standard opness of females with devil fruits... they usually have hax powers (e.g.: Perona, Sugar, Giolla, Brûlée, et. al.) , but never use it to its full potential. Mark my words, it'll happen to Bonney too (e.g.: "she doesn't have enough courage", "she doesn't have enough stamina", "she has a time limit, and the limit for Nika's future is even smaller", etc.).