I use a Vivosport, it’s pretty accurate it tracks within one or two beats of my Polar m450 when out on the MTB or gravel bike. Once in a while it drops out for a minute or so, it’s worst feature is stairs climbed, the wind seems to affect it a lot.
To make these type of watches measure correctly I have found, make sure there isn’t any hair under the sensor, need to shave, and I wear it on the inside of my wrist. If I put on the outside it’s all over the place but then I found the Scosche I have was worse on any place I put it on my arm.