Started a hike on the Apple Watch, selecting Phone as the recording device (because I was concerned about battery level on the watch). All during the hike the watch display showed speed, distance, etc as expected. When I returned to my starting point, checked the watch to stop the hike, but discovered the watch had run out of power. Launched the app on the phone, and was asked to authorize HealthKit (which had already been done previously). After doing that, there was no record of the hike on the phone. Watch interface (after charging up the watch) just offered ability to start a new hike.
I thought the point of selecting the iPhone as the recording device was that it should be recording ;-). I think the watch UI needs to clearly show if the phone does not start recording as commanded.
Also, as others have posted, it seems like there should be some way to recover the collected data from the watch app.
I really, really like this product, but of course losing data is never fun. Thanks for building a great app, and please keep improving it!