It is flawed because it's promoted as the best way of telling if you're overweight and so too many people take it as gospel. It's far too basic.
I am not disagreeing it’s basic. As I said, it is what it is. If you look at the formula it’s obvious it has limitations as it only involves height and weight; everyone can see this. It’s an indicator, a guide, useful for assessing populations but not as good for individuals. The flaw is with it being promoted as the best way of telling you you’re overweight; no one should be doing this. Who says it’s the best way? The school?