You just said it right there that Kante isn't good enough on the ball and part of being a box to box midfielder is being good on the ball and without the ball because you have to support the attack competently, as well as track back from central midfield and track runs or mark players come into the box then transition not only the ball but yourself towards the opposition box. I think at the very least, it's fair to say that Kante is a central midfield player who makes the most optimal impact on a football match as a defensive genius who intercepts, tackles, retrieves, screens his defensive backline, presses, closes down players, etc. His best actions are defensive and that benefits his team tremendously. If you're not able to maximize a player's strength, a Premier League winner and World Cup winner from the heart of the team (i.e. midfield), I will have my doubts as to "a problem with his playstyle".
Kante covers a lot of ground because he's pressing, retrieving the ball, closing down players all from a lower central midfield position throughout a match. Kante couldn't just sit back and sweep like Busquets or Carrick because he couldn't pass nearly as well as them nor does he have the ability to be a metronome with/without the ball.
Most tackles per season in the Premier League since 14/15/:
14/15 - Matic (129)
15/16 - Kante (175), Idrissa Gueye (144)
16/17 - Idrissa Gueye (135), Kante (127), Romeu (117)
17/18 - Ndidi (138), Idrissa Gueye (117), Kante (117)
18/19 - Ndidi (143), Idrissa Gueye (142)