In addition to my Apple Watch S4LTE, I have a Garmin fenix 6 Pro Sapphire. I actually love both watches, but I bought the Fenix for the SpO2 feature, along with it's "body battery" function, among other things. I can state that I have an iHealth SpO2 meter, and it's very accurate according to tests. After wearing the fenix for a few days to let it get "used to" my body functions, respiration, etc, the Fenix's SpO2 meter is IMHO very accurate. I've taken a measurement with the fenix on my wrist, and the iHealth on my finger, same hand, and they've read if not identical, within one unit (%) of each other. Of course, it does take quite a hit on the battery if you leave it on 24/7, but it's interesting to see the variation. It takes the battery from around 10 days with it off, to about 7-8 with it on for "Sleep Only", to around 5-6 days when on 24/7.
I suspect the battery hit is most likely the main reason for it not being on the AW.