BMI is fine, but the algorithm behind it is very simple and does not scale well.
It is accurate for people of average (belgian male) height (circa 1840), but people above or below average height get misleading BMI numbers.
People above average height get higher BMI number, believing that they are overweight even if they have very low body fat, whereas short people get much lower BMI even if they have more body fat.
The scaling issues are pretty well known, and these inaccuracies causes the whole metric to become less relevant.
And thereby the products that rely on it...
However, there are better alternatives, and I think Healthmate should support those:
There is a more complex BMI algorithm called "New BMI", which scales much better for height.
Here's a calculator for that: New BMI Calculator
Here's a mathematicians blog post about the algorithm: Blog post on New BMI
There is also something called a "Ponderal Index", AKA "Corpulence Index" which is simpler, but also scales well: Ponderal Index Calculator
I suggest supporting the "New BMI" as an option in the HealthMate App.
Users could switch to using the "New" algorithm in settings, and get their BMI calculations based on that instead if they prefer it over the old (default) BMI calculation.
A bit like the "Athlete Mode" setting feature I guess?
A graph for Ponderal Index would also be cool, maybe as an alternative or optional addition to the BMI graph?