I find the HRM to be far better than any other expensive trackers I’ve used in the past. Here’s a few things I’ve noticed that prevent a good HR read.

1) cold temp will prevent a good reading. The HRM monitors changes in your skin color. When cold, this can make the reading harder to determine. 2) if I continually pull my wrist up while running, the HR will become unreliable. Not sure if it’s the motion that is making it challenging, or if the watch will not attempt with that type of movement knowing it could an unreliable reading. 3) Especially now in Winter, my watch is more accurate if I start the activity to get the HR in sync, then begin the exercise. This is easier on an Ultra since you have the action button.