I've got both a garmin premium chest strap that I sync to my edge 800 and the vivismart HR.

the chest strap once I'm sweaty is significantly more accurate for instant heart rate readings.

the vivosmart is good for long duration average readings.

you can set the vivosmart to act as a HR sensor that reads over ant+ but it significantly reduces the battery life.

I've done that once when I forgot the actual sensor part of my chest strap on a ride.