For me, I compare him to Cleveland Williams. Great knockout record, very impressive looking fighter physically, enormous power obviously, but when he went against the best, he just couldn't get the better of him. When you dig deeper into their records you realize that there's unfortunately not a whole lot of depth there, apart from those losing fights against the top fighter of their era, so it's hard to rate them anywhere close to being an all-time great.
Maybe if that guy didn't exist, in Wilder's case Fury, in Williams's case Sonny Liston, he would have been. But it just wasn't meant to be for either guy. Wilder also never got that fight against Joshua, just like Williams never got the fight against Floyd Patterson, which may have been career-defining. In Wilder's case he was probably more responsible for that than Williams who was blatantly ducked.