You’re not wrong about the injuries but how many guys who played for both Carroll/Belichick or Carroll/Parcells (or all 3) had their best years under Carroll and his staff? Milloy is the only one I can think of.
Grier was a horrible personnel guy in retrospect but Carroll didn’t do much of anything to develop talent and his offensive scheme/staff left a lot to be desired. Even on defense, they didn’t really seem to know what to do with Brandon Mitchell or Tebucky Jones, and didn’t bother to try to develop Greg Spires (who had a nice career elsewhere). How much of that was on Carroll or Sidwell, who knows. But McGinest and a few other veterans underperformed under Pete and for a defensive guru and players coach, not a single guy who came into the organization while he was there became a core member of the first few Super Bowl wins (the aforementioned Mitchell and Jones did of course have key plays on the 2001 team).
I don’t think he was terrible and put more of the blame on Grier, but his main strengths - teaching and motivation - seemed to get refined significantly at USC.