^^ This - the ECG & Blood Oxygen levels are "good enough" for the public to consume and track, but perhaps not quite at the level of monitors used in medical settings to make care decisions.
And I didn't just check the bog standard 98% measurement but also tried to hold my breath etc.
On the contrary, the Apple Watch ECG is quite accurate, not quite enough to replace the system the doctor makes me wear, but enough that apple can get a general idea of any defects and tell you to go to the doctor. The "regular" hear rate sensor isn't anywhere near good enough to be taken seriously, but its leaps ahead my previous samsung.
I dream of the day where my Apple Watch can replace the myriad of annoying wires and sensors I have to wear to monitor myself. Even then, the Apple Watch has greatly helped me out when I've fallen and need assistance, and I've gifted several to my family just in case.
I'm using a $70 Amazfit right now but previously I had a Samsung, neither is very good.