In all fairness, had any one of KG, Shaq, or Perk not gotten injured during the playoffs, Doc would have won 2-3 titles. Still almost pulled off 2 titles if it wasn't for Ron Artest saving the Lakers in that brutal game 7.
Pierce was an immature player with flaws in his game that he picked up early on from his days with Antoine Walker. Doc was stubborn enough to get him to buy in, improve and get to the next level. Doc also was really the only guy who managed Rondo well enough to get him to mesh with his teammates and maximize his talents, at least for most of his time in Boston.
He developed Tony Allen, Perk, and Avery Bradley into key players on a title team, and even got good stuff out of Glen Davis and Nate Robinson. He also won coach of the year early on in Orlando turning around a mediocre Magic team.
Getting an MVP season out of Embiid and keeping the Sixers in the race as contenders with all the Simmons drama this year helps too.