BBC

Alot of what they collect are meaningless in the context of day to day health.

Pulseoximetry is a good example. Your pulse rate will go up and down as you exercise and rest, and your oxygen saturations may fluctuate. In a healthcare setting they're only really relevant when you're ill - they're useful to measure how ill you are or as a warning sign that you're deteriorating.

But they're not really useful as measures of how well you are because when you're well they will be normal. Constantly monitoring them is pointless.

As devices to guide exercise - seeing how active you've gotten for example - they may have some use. But for monitoring how well you are, not really much help unless you have a chronic illness like COPD or if they catch evidence of problems like heart palpitations.

The calibration and medical certification side is also an issue, but mainly it's just that it's a bit pointless.

Companies are throwing money at this stuff and trying to get consumers on board (because they think there is a lot of money in health care), but it's largely a waste of time and money for most healthy people beyond guiding exercise. And even then it's a bit dubious how useful it is.

Edit: also as you allude to, these are devices that help track you and harvest data. Getting people to essentially wear monitoring tags so the companies can harvest and sell data on where you are and what you're doing will make them money.