I've been pretty happy with my Garmin Vivoactive HR for cycling -- I too was worried that it would be wildly inaccurate, so I did a few rides with both the Garm

I've been pretty happy with my Garmin Vivoactive HR for cycling -- I too was worried that it would be wildly inaccurate, so I did a few rides with both the Garmin (using its wrist sensor) and my old Polar watch with a chest strap to compare them. They were very close to each other -- both on the bike looking at live readings and later looking at the charts at home. However, it could see it having less accuracy with an activity with more arm motion, like running. And I have no evidence that my Polar sensor was very accurate itself.