the exoskeleton is steel skin, the internal organs and bones remain that of a normal human.
at first i thought a harder skin would limit mobility, but it wasn't the case for sanji's siblings.
so no drawbacks. or the modification also gave them more physical strength to mitigate the reduced mobility, iirc by the time they developed their exoskeleton they were already as strong as full-grown men.
it's also makes sanji's sudden awakening feels kinda cheap, because for his brothers it took them years of constant progress and discipline to develop their gifts, while sanji awakened everything at full power in a matter of minutes.
ichiji/niji/yonji's gradual progress VS sanji's instant awakening.