BMI isn't a reliable indicator go gauge obesity FOR EVERY SINGLE PERSON. I would think most of us agree that the people like Tom Brady, Dwyane Johnson (the rock), or competitive body builders would be extreme ends of this scale.
BMI assumes you have an average amount of muscle. Any weight above that is assumed to be fat. So for someone like the Rock it treats a lot of muscle as though it were fat.
For average people BMI is a reliable indicator.