"Has a rapper ever had more support/hype and failed than Saigon?"
Hittman maybe?
Seriously, this cat had an album exec prod by Just, beats from Ye, beats from Preem, verse from Jay, major part (playing himself) on a hit HBO show, and has never lived up to the hype.
Granted, he had issues with Atlantic, but shit, this dude was getting in articles in magazines like Newsweek and shit and couldn't even make a dent.