What a bunch of you aren't getting about Brown and Tatum's defense is Brown is assigned the tougher player every time. Brown guards the big ball handlers/offensive engines. Tatum doesn't.
Smart and Brown cover the Cade Cunningham's, the Luka Doncic's, the James Harden's, the Jimmy Butler's of the NBA. Brown gets the tougher job. Tatum is usually on one of the weakest offensive players the opposition has. It allows him to roam and play great weak side defense.
But if the Celtics need the guy that runs the show stopped or slowed down, it's Brown and Smart that get the tough assignments.
For that reason, and his ability to guard 1-4(remember Brown on Siakam in the playoffs), I rated Brown as good as Tatum, just with different strengths. Horford too. That why I rate them all about equal in defense.