Even sasuke has an external power....EMS. he literally has someone else's eyes in his skull.
The same way that sasuke being an uchiha allows him to do that, naruto being an uzumaki allows him to house the bijuu(which is no easy feat).
Sasuke is using his brother's eyes to retain his sight for the mangekyou, while also enjoying the power boost it gives, as Sasuke was completely against using Itachi's eyes before he went blind. Though it is true he has itachi's power on top of his own now, it's still sharingan, no other separate entity lending the power, his and itachi's eyes became one, transformed, like madara and izuna's eyes, however naruto has kurama and the other beasts, all separate entities... thus, those entities are not his, they are a borrowed power.
And naruto is not the only one who can do so. Bee can as well, and I would argue people like orochimaru, sasuke, tobirama, hashirama, madara (proven, though obvious), minato (proven, but also obvious), kakashi, nagato, and itachi can also do the exact same feats as naruto. Naruto isn't the most powerful, he is gifted yes, just like the others listed above, but that is all. They all have their weaknesses, as itachi stated, "Every jutsu has a weakness". That goes for people and things in life, as well.