I’ve had the Garmin forerunner 935 for over a year now. The heart rate was so inaccurate initially that I emailed Garmin and they sent another one. My chest strap would read 150 and my watch would go as low as 38bpm!!! Unfortunately, the second watch was no different. After being in contact with Garmin numerous times they basically said that the sensor is racist 
jk! Buuuuuuut that it doesn’t respond as well to skin with low melatonin (ie. this white girl), which is even funnier because I’m sooooooo pale where my watch goes.
I got over it pretty quickly, and find it amusing jowadays when I’m dying on the trainer and my watch says 55bpm, ha! It does appear to be accurate during general gym weight lifting and every day activities such as walks, sleeping, chasing the dog around. It just doesn’t work for me with higher intensity work (140-200bpm), whether I’m sweating or not, or if it’s cycling or other high intensity workouts.