I've now had two Garmin devices with built in optical HR modules -- the Vivosmart HR and the Forerunner 235. It is a huge advantage not to have to wear a chest strap, but the Elevate HR module is not perfect. For God's sake, it's not a medical grade EEG -- its a device that can aid in pacing and effort. Properly worn, the Elevate HR module in these devices works pretty well. However, I have found both devices can give erroneous HR info under the following circumstances:
1) It's cold out, and the device is worn over your sleeve or exposed to the elements. In this case, it is likely you will see some high HR readings for the activity - anywhere from 30-40 beats per minute faster than you might see otherwise. Fix: give your device a minute or two to "warm up" -- start an activity and wait a short time before you hit "start" and begin timing. Also - keep your sleeve over the device when you are running, at least until the device (and your skin) warms up a bit.
2) There is too much slack in the tightness of the wristband during activity. The tracker/watch should stay in place on your arm. Not so tight that you are leaving tracks on your wrist from the band, but not so loose that it moves at all when you are running. A loose device can mean spikes at the start of the run - as high as 40 beats per minute faster than where you should be -- and spikes once the run is underway.
3) Conversely, during resting periods, let it out a notch. I find that if its too tight during resting periods, it might show a higher resting heart rate than what you are experiencing.
With a little trial and error, you will find what works best, and most accurately, for you. I also hope that Garmin continues to improve devices with Elevate HR modules so the recording of both active and resting HR gets more accurate over time.
Overall, I have pretty good results (as good if not better than a chest strap) during activities when I have the proper tension set on the band, as below: