Tanner Glass
kiwifarms.net
Off the top of my head, Obsidan has lost George Ziets (Mask of the Betrayer & New Vegas), Kevin Saunders (Sith Lords & Mask of the Betrayer), and John Gonzalez ( New Vegas, his most well known contribution to it being House). Brian Mitsoda also worked there for a spell but to be fair he didn't really contribute much before leaving.
According to Avellone, Urquhart and Parker are useless assholes. The former really fucked over Tyranny by funneling funds that Paradox gave them for it into Pillars of Eternity.
And on top of that, Sawyer was supposed to be "hands off" on this project - only working in an occasional advisory capacity, despite the fact he's likely the biggest "fallout game" resource available and probably should have been front and center on the project.