What are the limitations of using bmi as a measure of weight and overall health? how might muscle tone affect bmi?

BMI measures excess weight rather than excess body fat, therefore it doesn't take in to account the amount of muscle you have. This would mean that if you used BMI to judge your overall health, it would be false if you were very muscular. BMI may determine that you are overweight, but it is not necessarily correct as BMI is judging based off of excess fat rather than muscle.