considering its the most violent multinational sport in the world, boxers actually have quite a long carreer. I live in london but even i know you will be lucky to last 4 seasons in the NFL
It is pretty high on the list.
Just think about the top prospects that fail to deliver, in comparison to other sports.
The mental side of the game is so important and you can not tell how a guy will truly react until the going gets rough (or seemingly hopeless).
On top of that you need to be a great athlete.
Great athlete plus a fighters mentality is a rare combo