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A recent study shows that fitness trackers with integrated heart monitors, including the Apple Watch, are pretty accurate, but that energy expenditure - the red ring on the activity app of the Apple Watch - are not so reliable. The Apple Watch is better than some other devices, but still is off by as much as 25%-50% in either direction (fewer calories recorded or too many.)

See https://www.theguardian.com/technol...f-step-when-measuring-calories-research-shows or, if you really want to geek out, the actual study at JPM | Free Full-Text | Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort | HTML

To be honest, since the watch is measuring activity the same way every day, it's probably a relatively reliable way of tracking one day to the next, but people may want to rethink caloric food intake based on the activity reported by the health app.