Remember there is a guy that said Dembele is as good as Di Maria???
For a lot of people it's about optics and exposure to the media, due to his Man United exit and being in France for so long (and the fact that he doesn't score a lot of goals) Di Maria hasn't been in the spotlight nearly as much as he used to back in 2011-2016. Some people mistake that as him not being as good as other players that receive far more media exposure. For example, just by virtue of being a player for Barcelona a lot of people proclaim Dembele as an elite player/talent. It also doesn't help that Di Maria doesn't score a lot of goals and gets tons of assists, as his play style is more about facilitating playmaking than putting in the final ball.
He's not world class, but he is really fucking good especially during big matches. Really underrated player.