If you really think about it, BPA is a myth.
Once you factor in schemes, no two teams are going to rate a player the same. So it's just that team's opinion, and then their ability to get develop that player.
Every round, each team is going to take the best player available - take the Vikings with Christian Ponder - I'd be willing to be that whoever ultimately made the decision to draft him thought he would eventually be a very, very good QB - which would have made him, at that time, the best player available. It's easy for us to look back and say "Ha that team sucks at drafting". But the reality is, we don't know what any team's criteria is, we don't sit in on interviews, and we don't have access to as much tape as NFL teams/scouts do - and on top of all that, fans don't possess the same skills to interpret that film and project guys to the next level.
Every play in an NFL game, every player either wins or loses. Funneling that down, you could say that half the draft picks will succeed, and half wont. Figuring out why is the most complex part of it. Would Tom Brady be Tom Brady if the Rams drafted him in the 5th round that year? I think not. He'd be a JAG.
Could Ryan Leaf have succeeded on another team? He was a headcase, but it's likely that he could have.