Im not sure about them ruining Boumsong.He joined from Rangers after previously playing for Auxerre. Now the french or scottish leagues aren't exactly the strongest so he wasnt tested as much therefore his shortcomings weren't exposed. When he had to step up to the premiership or serie a, he couldnt do it. Also, take into consideration that he was playing alongside Bramble. Im not sure if Boumsong was ever a good defender to begin with. He had a good game for Lyon against Man Utd in the first leg though.
Emre, is injured alot but yea he didn't fit in to Allardyce's tactics.He still shows glimpses of what he can do when he plays.
as for Owen, his biggest mistake was leaving Liverpool when he did. Injuries have taken their toll on him as well. He isnt as quick as he was which was his strength. I would give him more time though.
Damien Duff, was brilliant for Blackburn and was one of Chelsea's best players the the first year they won the league. Then he got injured alot and seemed to lose all his confidence. He stopped trying to beat players and instead plays deep and plays simple passes. I think Mourinho ruined him. He reminds me of Kewell a bit. There were times when Duff would get the ball and beat 4 or 5 players. Injuries have fucked him up a bit as well.
Stephen Carr, once touted as one of the right backs in Europe. Again, injuries have fucked him up as well. He is nowhere the player he was.
In alot of cases, Newcastle spend big money on players who were never really good to begin with or are in decline.
It sayes something about a team when their best player for the past 2 or 3 (since Shearer retired anyway) is their goalkeeper.
Another thing, a lot of their players over the years are very injury prone. Maybe their fitness coaches are shit?