When I first bought my Garmin fēnix 6, I was really curious if the optical heart rate (OHR) sensor was as good as a strap. So I ran some tests, and it was just OK. The problem I had with it is perfectly summed up in this quote from DCR’s review of the Scosche Rhythm+ 2.0 Heart Rate Sensor:
And this is a perfect example of what I typically see for wrist-based (e.g. the Garmin Enduro) optical HR sensors outside cycling, especially in winter as I am now. They struggle there on sprints. If you scroll back up, you’ll see that for the steady-state portions it was fine. But these very intense 15-20 second sprints just aren’t ideal there. However, the Scosche does just fine, minus the few-second delay we see on optical HR sensors.
Which is to say, it’s OK for a lot of things, but not for what I use a HR sensor for on the bike; where better responsiveness is critical, to me, for pacing. That’s something of a preference thing though. If you don’t incorporate HR into your pacing strategy (or whatever), and you’re only interested in tracking stress (me when running or doing other non-bike exercise) and don’t want to bother with a chest strap, then the Rhythm+ 2.0 might be perfect for you.