According to reviewers, Mi Bands and generally all of Xiaomi's devices are fairly bad at sleep tracking. So much so that it's almost pointless to use them for that.
I know it’s fully anecdotal, but my Xiaomi/Huami (former Xiaomi sub-brand) devices have matched up sleep duration quite well with what I feel in the morning - and I’m quite sensitive to less sleep. I think it’s quite good for sleep duration, but I don’t rely on REM duration etc as I don’t have much use for it. When it predicts a good sleep score I’ve definitely slept well, though sometimes it gives a lesser score even when I don’t appear to have slept poorly.
(I’ve used a Mi Band 1, an Amazfit Bip, and currently an Amazfit Band 7.)
(I’ve used a Mi Band 1, an Amazfit Bip, and currently an Amazfit Band 7.)