Step counts as well as the fitbit. Heart rate is simple. How can it mess that up? Blood pressure is relative. You don't expect it to be accurate without a cuff. Same with O2. Those can show whether you're higher or lower "now" than you were an hour ago, a day ago, but they're not intended to be accurate on an absolute scale. Sleep seems to match up pretty well with snorelabs.

Now that I think about it, I wonder how well any of them keep time without syncing to their app.

>Heart rate is simple. How can it mess that up?

You might be surprised: https://www.cnbc.com/2016/05/23/study-shows-fitbit-trackers-...

That story is about Fitbit, which I don't have.

I check my pulse against a stopwatch during the same minute that Chinese no-name is counting my pulse, and the result is the same.

I mean, I'm not expecting them to pay a full $1 to get an RTC that keeps time up to 1s/month, but still, 45s/day is a lot, and it implies they really cheapened out on either design or components. I would mistrust even the relative measurements.