How reliable is the Apple Watch's blood oxygen sensor?

A separate question is how reliable the blood oxygen sensors actually are. For example, see https://www.fda.gov/medical-devices/safety-communications/pu...

> Many patient factors may also affect the accuracy of the measurement. The most current scientific evidence shows that there are some accuracy differences in pulse oximeters between dark and light skin pigmentation; this difference is typically small at saturations above 80% and greater when saturations are less than 80%. In the recently published correspondence by Sjoding, et. al.External Link Disclaimer, the authors reported that Black patients had nearly three times the frequency of occult hypoxemia (low oxygen levels in the blood) as detected by blood gas measurements but not detected by pulse oximetry, when compared to White patients.