About Sabi Sabi no Mi
The Sabi Sabi no Mi is a Paramecia-type Devil Fruit that allows the user to cause metallic items to rust and break, making the user a Rust Human (サビ人間, Sabi Ningen?). It was eaten by Marine Captain Shu.
The primary ability of the fruit, so far seen, is that the user is able to rust metallic weapons such as swords through touch, causing it to break apart. Doing so gives the user somewhat of an advantage against sword-wielding opponents.
In terms of weaknesses, it has the standard fruit weaknesses and can't protect from fiery projectiles, as Usopp easily beat Shu in spite of Shu's advantage over Zoro.
Strengths and Weaknesses
The primary ability of the fruit, so far seen, is that the user is able to rust metallic weapons such as swords through touch, causing it to break apart. Doing so gives the user somewhat of an advantage against sword-wielding opponents.
In terms of weaknesses, it has the standard fruit weaknesses and can't protect from fiery projectiles, as Usopp easily beat Shu in spite of Shu's advantage over Zoro.
The following events are non-canon, and therefore not considered part of the canon story.
Shu demonstrates his immunity to metal weapons in the anime.
In the anime, it has been shown the powers of the fruit give the user a permanent defense against swords and other metal weapons. A sword simply touching the user's body will immediately rust and disintegrate before a cut can be made. It has also been shown in the anime that the powers of the fruit can also affect the human body.
However, if used on a living thing, the rusting effect seems to be reversible for as long as it does not entirely oxidize the entity in question, since Zoro was seen reverting quickly back to normal after Shu's grip on him was thrown off by Usopp's projectile. In addition, it seems that there is a limit to how much metal the user can handle, as Shu was easily knocked away by a large metal colossus controlled by Douglas Bullet. Other than all that, the user is also susceptible to the standard Devil Fruit weaknesses.
Concludes non-canon section.