Usefulness of the Apple Watch Series 5 to a Cyclist: does it have electric sensors instead of, or in addition to, optical heart-rate sensors?
I get far better readings from my watch during running and cycling than I did with either Garmin or Wahoo chest sensors. However, since 2017 I’ve not even tried a chest sensor, so perhaps they have improved faster than the Apple Watch sensor has. 

The watch is less likely to slip for me with the Velcro band than the elastomer band, but even the elastomer band is more accurate than I need for 6 hour cycling workouts on bumpy gravel roads in windy conditions. It also does more - so I’d rather skip the chest band entirely for convenience and comfort. I did love the chest bands since the early 90’s due to them being the best I could get for exercise data collection. 

All three vendors I list are superlative in my experience, as is Suunto, if you need excellent data on your heart rate while exercising. The watch sensors are more than adequate for me so I wanted to chime in with my experience.