At this point in time, I think you’d be hard pressed to use the Apple Watch as a true fitness tracker. You can use an old Garmin Forerunner 735 watch to record power, cadence, speed, and heart rate. HR can be recorded from the sensor on the watch or from a strap. If you wish you can also broadcast the HR signal from the Garmin 735 to ANT+ compatible devices. Can’t do that with Apple Watch. I’m sure Apple knows that there are other more capable fitness watches out there. But their target audience is more mainstream than us Strava (or Garmin Connect/Polar Flow) obsessed athletes.