I have the Garmin Vivoactive 3, which gives overall sleep + breaks down sleep into light sleep, deep sleep, REM, etc. My feeling is that the overall sleep number is pretty accurate, but I don’t trust the break down. The newer Vivoactive 4 which also has a SPO2 sensor, might do a better job on overall sleep tracking / breaking down sleep into phases.
For an interesting listen, the Sigma Nutrition podcast just had an episode on sleep - 312: Allison Brager – Sleep Architecture, Chronotypes & Rescuing Performance. In it, Dr. Brager says that Whoop & Oura Ring do a good job of tracking sleep phases. In this podcast, Dr. Brager came across as a pretty straight shooter, but I’d never heard of her till listening to this.