I believe he's more like a classical #10 than a modern #10. The likes of Silva, Eriksen, Kagawa, Ozil, Cazorla, Wilshere, Fabregas, Oscar, etc. drop deep and drift out wide, always looking for the ball and trying to dictate the game. These sort of players have a strong influence on a team's tempo, pace, passing rhythm and range, etc. Mata, on the other hand, has rarely, if ever, dictated play. More often than not, he hovers in and around the final third, waiting for someone to supply him the ball so that he can do his work in the final third. This is a reason why he has excellent goals and assist statistics whilst not being the most influential player in our team as modern playmakers normally are.
Given that he's not that dynamic, he's always at his best when he has dynamic players around him. In Chelsea, for example, he had Hazard, Oscar, Drogba, Torres, and Ramires, all of whom offered good movement and dynamism, which allowed Mata the chance to do his thing.
He's not really a midfield playmaker nor is he a modern #10. He's more of a final third player, and he's at his best with dynamic players around him.