According to most on-wrist HR device reviews I've seen (for example, dcrainmaker), the on-wrist devices work just fine but filter 'noise' too aggressively to accurately capture your abrupt heart rate fall when doing intervals. For most people, this is 'good enough'. I have an AppleWatch2, Fitbit Charge HR, and a Suunto Ambit that uses a chest strap. To be honest, the chest strap is unreliable enough that I don't think I'd say that system works better than the on-wrist devices.