I think he is mostly underrated because well....he didn't really do much. Here's a short breakdown of his abilities:
Spores: Able to drain chakra and give it to allies. This jutsu was undetectable by kage level ninja at the summit.
Earth traveling jutsu: Zetsu has shown the ability to merge with the earth and quickly travel from place to place. The level of this jutsu is interesting because even some kage-level sensors were unable to detect him infiltrating the kage summit until he chose to reveal himself. He also can use this jutsu for guerilla style hit-and-run tactics and ambushes as he did against the mizukage and her forces in the war.
White clones: He has shown the ability to steal and copy someone's chakra, making an exact clone, though much weaker than the original. Copies even unique abilities like kisames shark transformation. Even though they are weaker, I think this is the best clone jutsu and enough should be able to overpower stronger ninja.
Wood release: Zetsu was also shown using this Kekkei genkai, though a much weaker version. Still useful nonetheless. We really don't know what other wood release jutsu Zetsu has.
Sensory abilities: I think when he was alive (not sure if he's dead though) he was the best overall sensor. Narutos sensory abilities primarily detected bad intentions. And again he was able to hide from some of the strongest sensors.
Splitting ability: Zetsu has also shown the ability to split so both halves can act independently.
I think he is best fighting when both halves are in close proximity but split, so they can back each other up. Add his unique clones and spores to the equation and I think Zetsu makes a formidable opponent. The more I learn about him, the more I like him for his uniqueness. Probably my 2nd favorite akatsuki member after diedara. Since he has so many clones and such, I'm curious to know if he's fully dead or not.
Spores: Able to drain chakra and give it to allies. This jutsu was undetectable by kage level ninja at the summit.
Earth traveling jutsu: Zetsu has shown the ability to merge with the earth and quickly travel from place to place. The level of this jutsu is interesting because even some kage-level sensors were unable to detect him infiltrating the kage summit until he chose to reveal himself. He also can use this jutsu for guerilla style hit-and-run tactics and ambushes as he did against the mizukage and her forces in the war.
White clones: He has shown the ability to steal and copy someone's chakra, making an exact clone, though much weaker than the original. Copies even unique abilities like kisames shark transformation. Even though they are weaker, I think this is the best clone jutsu and enough should be able to overpower stronger ninja.
Wood release: Zetsu was also shown using this Kekkei genkai, though a much weaker version. Still useful nonetheless. We really don't know what other wood release jutsu Zetsu has.
Sensory abilities: I think when he was alive (not sure if he's dead though) he was the best overall sensor. Narutos sensory abilities primarily detected bad intentions. And again he was able to hide from some of the strongest sensors.
Splitting ability: Zetsu has also shown the ability to split so both halves can act independently.
I think he is best fighting when both halves are in close proximity but split, so they can back each other up. Add his unique clones and spores to the equation and I think Zetsu makes a formidable opponent. The more I learn about him, the more I like him for his uniqueness. Probably my 2nd favorite akatsuki member after diedara. Since he has so many clones and such, I'm curious to know if he's fully dead or not.