I'm actually at a point where I'm 99% sure that handicaps are exisiting. It's the main reason why I can win with a ~70 overall rated team against a ~85 rated team, you just notice that your passes, the players movements, and finishing, become almost perfect and free of error, while the opponent struggles to properly string together a few successful passes with far superior players. They will still have the advantage of more pace and strength, much more shotpower and better goalkeepers to just name a few, but these alterations make the match basically even.
This then becomes a problem in ultimate team, where there is no point in buying better players because it doesn't increase your chance of winning a game.