@dawgs2014 can correct me if I'm wrong, but I think the criticism of defensive rating is that it estimates how many stops a player gets based mostly on steals, blocks and rebounds. But of course these are imperfect proxies for how many stops the player actually contributes. The inherent flaw is that one could in theory accumulate a ton of steals or blocks or rebounds (and thus have a good defensive rating) yet still be a poor defender in that they allow the opponent to score a ton of points. Conversely, one could never get a single block or steal or rebound and be an excellent defender yet have a terrible defensive rating. The point of defense is to prevent them from scoring, not to block or steal the ball. Of course, in reality, steals and blocks and rebounds often probably do a decent job estimating who is doing the best job defending.