Few months ago I wrote an article about dual wearables. Point is, that while Apple and Google (Alphabet?) go for fashion markets with their wearable technologies, and for very good reason fitness trackers like Fitbit, Basis, Withings et al. can go for precision.
How to get precision? Well, one of the options is to have more than one device. Have one on each wrist, or one on a wrist and one on ankle. My small scale tests with Arduino based "wearables" showed that majority of erroneous data about overall activity, body temperature and heart rate can be eliminated and cleaned up if there is more than one source of data. Which is pretty obvious, but being an engineer in mind decision to order few sensors, soldering kit and Arduino came easy.
So yeah. While Apple and Google go for fashion. Fitness tracking companies can go for precision that Apple Watch and Android Wear are not ready to commit to.