The optical ones are fine for sleeping and sat at. Desk, they are complete junk while actually running. I have the scosche Rhythm+ which is supposedly the best optical you can buy, plus a Garmin 935 which is supposed to be the best watched based optical. Both are hilariously wrong.
Typical problem is cadence lock, so will just report 180 even on a recovery run. The 935 is worse because it locks on to cadence but doesn't seem to trust so knocks of some values ,will report 165 or some such for long stretches when it should be around 145. This is made really obvious running up and down hills, the 935 will reporr165-180 when running easy downhill when my true HR is about 135 because the added impact forces make it locks on cadence. Yet when I go up a hill it will get the true HR at 155-165 when im working harder.
And the lag is so high it never properly records intervals.
Stay well away of optical heart rate for training. It's great for checking testing heart rate though. Chest strap is much better. Not without flaws but these are much rarer and more obvious.