poor naruto, he has to master sage mode and nature chakra of which from his generation only his master jiraya barely mastered it, orochimaru barely learned it and wtf science'd it into the cursed seal, and kabuto learned it, cuz well he's a super genius. The only other guy we know is dead hashirama and he's like nearly a super god.
then he has to learn how to control the demonfox in his belly, which seems like only 8tails and 2tails can do, then he has to fight so hard and so much that the most evil biju decides he wants to work with naruto. only then could he be in biju mode and sage mode, he's basically almost got the power of hashirama, currently his power level should put at least 2 kage's to shame, he's fricking powering up a bunch of weakass ninjas too.
then saskae just goes, oh, i have these eyes, gogo susanoo, oh i've used this cursed seal for a while, gogo juugo, and bam he's on equal footing with naruto again.