Not having to set anything up or say I'm going to sleep is perfect for us, and the results seem to generally be in line with reality. I don't believe it tracks restless legs. It's really comfortable with the sport loop band.
From the limited research I did a few months ago it seemed as if the additional accuracy derived from a dedicated device was outweighed by the faff of setting it up + the additional utility of a multifunctional Apple Watch / Fitbit-type device. It felt like a single-digit % gain in accuracy over Apple Watch, which had a huge % gain in utility.