Sleep trackers can be inaccurate, and there is a surprising lack of testing and research on the subject. This research paper gives an overview of some of the research, and there have been some small studies, like this study in San Diego and this study in Nebraska that compared some commercially available trackers with polysomnography (PSG), which is the gold standard sleep study method (it records brainwaves, heart rate, blood oxygen level, respiration, and movement).
You might reasonably expect sleep trackers with more sensors, capable of recording similar data to PSG, to be more accurate, but manufacturers also develop their own set of proprietary