BMI is fine as an indicator of "are you fat or not", UNLESS you are a very fit athlete. However being fat and being unhealthy aren't 1 to 1, contrary to popular bias and stereotypes. I've known plenty chubby and healthy people. They might not run marathons but get on with their lives just fine. Certainly it can raise your chances of being unhealthy but it's certainly not 1:1 as many online "experts" seem to claim.