Board Thread:Naruto Gaiden/@comment-25645405-20150711211206/@comment-734582-20150711221643

It's explained that the Zetsu are what remains after Infinite Tsukuyome. I don't think having your chakra sucked out for decades and then turned into a half-plant clone makes you strong. Spiral Zetsu was strong because he used Yamato, which is why he was able to summon considerable abilities like the giant wood statue. However, most Zetsu have only fairly weak powers. Zetsu's strength was his ability to spy and dispose of bodies. Not his fighting prowess which the Zetsu simply did not have.

Orochimaru clearly has switched to a new body in Gaiden which is why i doubt Zetsu would give him immortality. As to why he doesn't use Sharingan, we don't know. But judging from how Shin worked out, i think he was actually working on turning Shin into an acceptable Uchiha substitute.