DC Rainmaker had a really good review of the original Apple Watch, with great analysis of the two heart rate meters (there is an optical one used during exercise and an infrared one used for normal, non-exercise use.) I suspect that AW2 is the same. tl;dr: the heart rate meter used during exercise can be all over the place. Sometimes it's right on. sometimes it's all over the place. And, as he says, if you want to use a high-quality HRM, you can pair the watch to a BT model to get better accuracy.
https://www.dcrainmaker.com/2016/02/apple-watch-review.html
I don't use the workouts app on my watch while I run (I use a Garmin watch instead), and the AW doesn't seem to track my heart rate while I exercise (every day that I exercise, there is a gap that is almost exactly as long as my run), so I can't say if it is accurate or not. Even while I was out with the snowblower this morning, there was exactly zero heart rate readings.
(Well, I did do one treadmill workout with the workouts app a month ago, and I just looked - it had my max heart rate at 130 - probably right.)
The non-exercise heart rate monitoring looks pretty good to me, though I haven't taken my pulse while the heart rate app is running or anything.