Naruto was actively resisting Kurama there too, remember? And that was just a TINY amount of chakra, no where comparable to the full beast.
Actually, Nagato quite handily defeated KN6 with his Chibaku Tensei, and was prepared to make an even larger one for full Kurama there remember?
Perfect Susano'o can only cut mountains in half. Remember, Perfect Susano'o only clocks in at 132 Megatons of firepower from it cutting two mountains in half. A standard Bijudama from one of the lesser Biju vaporizes mountains leaving nothing but a 6-10 kilometer crater in its place, which requires 4 gigatons of power. Biju Mode Naruto though? He goes over a TERATON of power, he can wipe out small countries with his Bijudama.
Biju Mode Naruto is superior to 100% Kurama too. Biju Mode Naruto can use ninjutsu, clones, and use Kurama's power far more effectively since he doesn't throw it around randomly.