I tried the UP3 and Emfit QS alongside the Peak for sleep tracking. They were all pretty good on time to bed and waking time but when I analyzed the sleep stage categorizations I found such bad agreement that I'm forced to throw up my hands and stop trusting any non-EEG device's ability to guess at true sleep stage.
Attached is a graphical comparison from a single representative night. Other nights I eyeballed were similar.
In the case of this night, all 3 devices agreed on sleep stage only 19% of minutes from start of sleep to final waking. Emfit agreed with UP3 44% of minutes. Peak agreed with the other 2 only 32% and 35% of minutes.
By comparison, they had reasonable agreement on just awake vs. asleep (with all 3 agreeing 73% of the night, and Emfit agreeing with UP3 88% of the night). So I'm giving up on sleep stage tracking for now.