Well, sometimes, an attacking player can have a terrible game but just be very decisive.
A midfielder is more than raw stats imo. You have to see him play.
Example, you are Barça fan. You very well know the all around game of a Messi. He does score, he does assists, but he also does some marvellous passes that can create the situation that will lead to a goal but won't be quatified by an assist or key pass.
Then, you can have someone else that is not very present in the game but is very decisive in goals and assists. From that, you could conclude they are equal or that the second guy is better.
There is a reason why PSG did better in CL when Marquinhos took Rabiot role.