Since BMI can't differentiate between fat and muscle, it breaks down for people who are very muscular. That said, most people are sedentary and hardly even exercise, so BMI is a good approximation. The people who are very muscular are probably well informed about this caveat that no buff bodybuilder thinks they're the same as an overweight person just because their BMI is the same.

Great point. This comment reminds me of Mary Cain, who was fast and dominant until she joined Nike and Salazar tried to get her to lose weight. It became clear the lower weight impaired her health and performance.

It bears repeating that BMI at an individual level is at best a hint that something is wrong (and never that someone is healthy).

Another point is that you can be good athletically speaking and yet have too much body fat to be considered healthy. An extreme example is that of professional fighters in open-weight categories.

Or BMI is a bullshit health metric. Ask any bodybuilder or powerlifter.

It is bullshit, but for most average people with average levels of exercise (or no exercise at all, sedentary lifestyle is pretty common) it's good enough to tell if something is wrong.

It's not A1C or something that's actively tracking your success and management in the long term, it's just the kind of thing where you look at it, look at the person, and say "is this a problem?" and usually it's clear.