I have a Garmin vivosport wrist based monitor. Ive tested it back to back with a garmin chest strap on the trainer and during a couple of Cx races and I would have to say I’ve found it bordering on useless.
Typically it will show about 100 to 120 bpm whilst my chest strap is showing over 160, and can often drop back to about 80 mid interval.
It’s useful for recording distance and resting heart rate, but that’s about it in my experience.