Pretty much only true if you are a body builder or someone else with an unusual amount of muscle mass relative to your height. For the vast majority of people though BMI is fine.
There is little point of doing a study based on BMI when what you want is body fat. BMI is imperfect. It’s fine when you want to give people a quick metric for them to assess their fitness by themselves. People who are extremely fit with a high BMI already know they are extremely fit after all. It’s not fine when you want to analyse the effect of different life styles.
