I'll take your word for it. But he never actually plays midfield when I watch him. He's either playing left forward or second striker for most games. He is also competing with Bale, Isco and Ronaldo for a starting position not Modric, Kroos and Casemiro. Martial plays as a winger/wing forward. Sure, semantically speaking they play different positions. They both play in front of the midfield and hence it's quite fair to judge them based on their productivity I feel.