The Apple Watch is quite accurate (according to Apple's own research, at least), and it doesn't guess or map to typical patterns: https://www.apple.com/health/pdf/Estimating_Sleep_Stages_fro...

From other anectdata the apple watch seems to just as bad as everything else.

We just don't seem to have any idea how to track sleep quality even with better devices than just a watch/whatever. Even a binary, is the person I'm attached to sleeping, is hit and miss. And with that in mind you can be pretty sure that the sleep stages etc is even worse.