Scouting and evaluating football players is not and can never be an exact science. You can compile all the statistics and vital measurables you want, but ultimately they will merely predict rather than guarantee whether or not a player is going to be able to be a success at the highest level of the profession.
One of the best tools that evaluators try to use to gauge how good a player might be is to isolate how they perform against competition that they also believe to be of a high level and use that as a touchstone for their analysis of what that player is capable of doing.