IMO his physicality is overrated. He gets pushed off the ball often. He wins aerial duels because of his height, and because he is generally going up against players shorter than him by a few inches. At 6'4, 185lb he is dead center in the 'healthy weight' category - for a normal, non-athlete, 164-204lbs. IMO, controversial opinion here, he gets characterized as being strong and physical because he is black, due to unconscious bias.
He wins us the ball an average of 1.4 times per game, when factoring in the amount of times he commits fouls to give it away: 1.8 Tackles/game, 1.3 Interceptions, 1.7 fouls. Compare this to Trapp's 3.6 times (2.5 tackles, 2.2 interceptions, 1.1 fouls), or Godoy's 2.4 wins per game, or Laba's 4.1 -- all DMCs from teams that finished within 3 points of Columbus, you start to see why he's not effective on defense using that statistical marker.
When you factor in passing on the above (Tchani 83.9%, Trapp 88.1%, Godoy 89.4%, Laba 85.2%) he starts to look even worse - again, these are DMC/CDMs for similarly bad teams, and he doesn't match up to those.
As mentioned he's inconsistent, and you can't be, as a CDM. If Wil Trapp played 1/3 of his games like Tchani plays 1/3 of his games as well, we would lose up to 2/3 of our games, depending on how much overlap there is. CDM/CM is not a position that can be inconsistent, because it is so incredibly vital for the rest of the team.