If you set the attributes of all players to "15" then it means that the defenders in your testing league have the attacking attributes such as "Finishing", "Off The Ball", "Dribbling", "Long Shots", "Flair" and some other attacking attributes at very high level that they would never had in the real game and the same time the attackers in your testing have the defensive attributes such as "Tackling", "Positioning", "Marking" and some other defensive attributes at very high level that that they would never had in the real game so your testing league doesn't simulate the real game well and it might be that some tactics perform very good in your testing league due the configuration of your testing league but in the real game they performs poorly.
Also, there's no real players in the game that have 20 for the left and right foot