Studies show that Fitbit and Oura sleep tracking is very inaccurate. For example, the measurement of being awake agrees with polysomnography half the time. I think the consumer device sleep trackers are a measure of people's willingness to believe in anything.
The best consumer review I have found of the Oura ring:
In the comments, the author's husband Chris states: “Who knows how accurate the ring is, but hopefully it’s at least consistently wrong in the same direction every time, so it can still help us see if we’re improving…” This may be true. After all, it's measuring something, right? The studies above measured a single night's sleep of many subjects, not one subject's sleep over many nights.
My personal experience, is that even if these devices aren't accurate, they give you an idea of when you fell asleep, and when you woke up. For those that can't keep a regular sleeping schedule, that can be helpful upon waking, to know if you should keep sleeping, so your day won't be miserable.
I have a fitbit, and I find it uncomfortable to sleep with. Sometimes my hand is numb because the bulky-thick watch has been pressing against my wrist. I've used it to track my sleep for many months, and am glad I tried it out, and *feel* like I have a baseline for future measurements, however, I'm unlikely to continue to use it for sleep tracking, unless it's to compare with a ring I purchase in the future.
My experience with the Fitbit, is that it is not accurate at measuring interval training, like sprinting. It doesn't register the high heart-rates very well. I've read it has trouble taking readings if the watch is being bounced around a lot, like when you are running. In cycling I think I've had better readings (this is my personal impression, as I didn't make it a responsibility to comb through the data to give a better conclusion).
The one thing the rings can't do, compared to the Fitbit, is vibrate to give notification of a calendar event, message or call. Unfortunately, you can't set Fitbit's default handler for calls. So you will only get notifications for calls to your cellular number, and not any other number like Google Voice. Neither does it recognize Google Voice as a messaging app. Recognized messaging apps include the default SMS, Hangouts and WhatsApp.