Yeah I don't know about ECG but I checked my (non apple) watch with a real finger-clip SpO2 from the doctor and while it's pretty accurate under highly controlled situations (sitting still and positioned perfectly) it's really lousy under real-world conditions, moving around etc.

And I didn't just check the bog standard 98% measurement but also tried to hold my breath etc.

I can't speak for the Apple Watch, but the O2/BPM readings on some other watches are often dropped/delayed with the operating system taking priority. I was trying to use the heart rate data for a pet project and found its practically useless, on Samsung's watches its just enough data to show a few fancy numbers and thats about it.

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.

ECG is a relatively easy thing to measure, so that single axis they measure is quite accurate. But the conclusions one can make from that is limited (you basically take a video from a single point). Doctors use 12 axes instead to get a real-time, 3D vector of the heart.

Yeah I heard Apple Watches have better sensors, I wonder though. I can't use them myself at all as I can't use iOS (too limited for my requirements).

I'm using a $70 Amazfit right now but previously I had a Samsung, neither is very good.

To be honest, “real finger-clip” sensors are similarly fuzzy in their results - after all they work the exact same way. Maybe using percentiles is not ideal as it gives a sense of more accuracy than what makes sense.

They don't work quite the same way. The finger clips shine through the entire finger. The illuminator is on one side and the sensor on the other. The wrist-based sensors rely on reflections through the skin which is a lot more iffy.