I agree with the chemistry point, or just the songwriting working. There's been many so-called supergroups or sideprojects that sounded great on paper, that didn't really do much for me.
I actually think what works better sometimes is if you have a great songwriter who writes most if not all the music in his or her primary band, and then wants to do music of a slightly if not somewhat dramatically different style.
And then this songwriter ends up being joined by other musicians they respect.
I guess something like The River Empires is a good example of this for me. Jessy Ribordy is the songwriter composer, making music that is a fair amount different than his primary band Falling Up being that it was more rooted in Bluegrass, Folk and Orchestral music, as opposed to more heavy/hard(core) rock. And he collaborated with Casey Crescenzo of The Dear Hunter.
The Dissociatives is another actually, with Daniel Johns collaborating with Paul Mac. The Dissociatives music is a little similar to the 2000's Silverchair in the use of layered production and harmonies, but it has a lot more emphasis on catchy hooks and melodies.
But in both of those cases, it's not like it comes across as having too-many-cooks-in-the-kitchen.
I dunno, some of it could come down to a musician and their influences. Like take Between the Buried and Me. It's pretty obvious the band draw from more than just Metal and progressive Metal; stuff like Classic Rock/Classic Prog namely. The members have done other projects, but I don't recall any one of them showing that influence directly. I could see Paul Waggoner or Dustie Waring doing a rather retro almost Led Zeppelin, Pink Floyd, Queen-ish like project at some point. Maybe a little like Presto Ballet maybe? something in that style, that if the songwriting was good, and perhaps it would be a collaboration with someone they know and respect (Casey Crescenzo maybe?, or Joey Eppard of Three maybe), I could see myself and others really enjoying.
And I actually would be more intrigued by that than honestly any of the other side projects they've done, and actually BTBAM's current music.