Doesn't need to be accurate, just consistent in its inaccuracies over time. It's about changing behavior, not diagnosis.

Being consistent is almost as good — essentially as good in fact.

I have a glucose meter that’s quite wrong, but the shape of its curve isn’t bad (the error isn’t linear, but is consistent— around 100 it measures about 82; at 150 it measures 130) but now I’m used to it it tells me what I need to know. And the strips are cheap.

I don’t need the watch to tell me if my heart rate is too high but it’s useful if it tells me my heart rate was about the same the whole run.

I have an Apple Watch and iPhone and use all the health tracking stuff. I’m a little bit suspicious of the accuracy of sleep tracking, but so far I haven’t had any experiences to suggest that it’s not reasonably accurate.

And there’s no reason to believe that heart rate and step counting isn’t extremely accurate—seems fairly trivial to measure.