IMO, Young was brought in to compete for the backup QB position to see if he can be a credible backup as well as a wildcat QB. This would enable the Bills to replace both Thigpen and Brad Smith with Young, saving significant $$$ (Young is guaranteed about $1 million with another $1 million in incentives) and a roster spot. I think that Young makes the Bills if he can replace both Thigpen and Smith. He might make the squad if he can replace Smith in the wildcat, but that's iffy.
The biggest problem with Young may his inability to learn quickly and/or his attitude. Reports out of Tennessee said that they had to "dumb down" their playbook for Young. Maybe he wasn't serious about studying in his early years, and maybe he's grown up.
Now, the Bills don't run a complicated offense but they expect their players to know their assignments well. They expect them to work hard, too. There are a lot of guys who were low round draft picks and UDFAs who have worked their tails off to get into the NFL, and I don't think a diva personality is going to fit the team.