Heyo! Couldn't tell you why its underused, to be honest. It looks fun. But basically it is the kekkei genkai of whatever clan Jugo is from said to be unnamed. Probably extinct since we went the entirety of the series without ever seeing any ability like Jugo's from anyone other than Jugo. It allows the user to manipulate senjutsu but manifest it physically, outside of the normal sage jutsu you would see maybe Jiraiya or Naruto use. Physically, they boost their physically strength and mold and transform their bodies outside of what is normally able to be physically done. This ranged from extending weird, skin and bone like wings from their bodies to remolding and reshaping their humanoid form into that of like maybe a dog to make them run faster. I don't know if that is actually a thing but, just saying, they are capable of altering their bodies a heck of a lot. This is due to their constant, passive intake of senjutsu chakra, making their bodies different from that of a normal shinobi. I believe this is where Orochimaru developed the premise of his Cursed Mark techniques, from Jugo's blood/DNA. Which is why you see Curse Mark level 2, their bodies take on major transformations. Such as Sasuke growing wings and having black eyes or Jirobo looking like a strong, red-colored demon. As far as the RP goes, same rules apply. However, I believe Sage Transformation users are immune to normal genjutsu B rank and below. This is not counting visual genjutsu. Furthermore, the further into transforming you go, the more mad and insane the user may grow. This seems to be more cosmetic than applicable in battle but there's that.
That's all I got.