>>180660790>>Buu is a lazy retard who's never available when he's needed
I honestly think that Buu was kept out of Super entirely because he trivializes every single conflict that wasn't Beerus.
Freeza was warned to stay away from Buu in the same breath as Beerus. Buu would have easily wiped out the army and one-punched Freeza before he could go golden, as alluded to by Krillin ("we'll be fine as long as Buu's here... wait, where's Buu?"). Even if he did go golden, Buu could have turned Freeza into candy. Freeza still couldn't sense ki, so he might have been completely punked by Buu from the sidelines even if he thought he killed him, not knowing about regeneration.
Buu would have ripped through basically all of Champa's fighters. Maybe not Hit, but his pressure-point attacks wouldn't work on a wad of bubblegum and it's ambiguous how strong he even was before Goku let him overclock himself and go full-power. In any case even if Buu couldn't beat him, slaughtering the other four would leave Vegeta at full power and Hit wouldn't be a problem then.
Buu could have easily dealt with Zamasu. Like, easily. Beat him up and then absorb/candify/fat-restrain him. Maybe not Black, but that one side of the equation would have been snuffed out pretty fast, leaving no one to save Black when Vegeta starts beating him to death in their first encounter.
Buu is ridiculously overpowered in the tournament of power; almost impossible to knock out of bounds (stretchy arms, shapeshifting), basically impossible to knock out, endless stamina with no need to reserve himself, turning people into inanimate objects and chuckin' em wherever he wants, maybe even absorbing them, healing teammates, telekinesis etc.
Unless Jiren is there to go "this guy is a problem" rather quickly, literally no other enemies would get a drop of screentime to do anything, because Goku and co. wouldn't even get tired.