I definitely give him some credit for gathering the pieces, but as you point out he didn't figure out how to use them together very well.
I guess most of it stems from my bias as fan of Plum, but I feel like he mismanaged her at every step for what, 4 years? It wasn't as bad, but I felt like he did similar with Young. Both of those players thrived instantly once he was gone and Hammon let them be the players they are and utlized their strengths.
Having two potentially dominant posts together can be a confounding problem, he wasn't the first coach to fail at it, but the Wilson/Cambage pairing certainly didn't flourish together.
Just generally speaking I think during his tenure players' roles either weren't very well defined, or were changed constantly which is really hard to adapt to. And like I said earlier he didn't play to their strengths in my view.