In my opinion, he is a very strong character, if only he wasn't betrayed by samehada.
-Being able to heal a severe wound in only a short amount of time
-His sword being able to suck lots of chakra
-Being able to fuse with samehada, and cast a giant blob of water, which he can drown his opponent
I personally think that naruto before he has KM will have a hard time on him, since his rasen-shuriken would be easily stopped
,However his weakness lies in the ability to fight against
-Kamui ( He's slow, obito could've easily suck him in )
-FGT ( Wouldn't be fast enough to attack )
-Sage's chakra ( He'd turn into a stoned frog like one of pain's body )
Since he can't absorb them with samehada or heal from it, since it can win a fight in 1 hit.
Please correct me if I overrated him.