he doesn't have kurama's yin chakra which defines the form of the chakra,plus this mode is way better than bee's it's only chakra that envelops naruto not an actual physical form,that means it's like some sort of susanoo,also if he's hit by amaterasu he cannot be harmed by it and if he drops the BM the attack will dissapear(unlike bee),the lack of yin chakra made him able to not be limited by the body of the fox and the chakra more volatile,he can expand his tails and turn them into hands like he did to the other bijuu