TLDR: For weight lifting, yes. They are universally inaccurate when compared with a chest EEG strap.
TLDR: For aerobic activities like spinning, the answer is mixed. If you want the best data, use an EEG chest strap like the Polar H10.
In the following graph, a weight lifting session done by Rob ter Horst, a postdoc at the CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna, Austria, aka The Quantified Scientist. While his conclusion was that the heart rate tracking on a popular optical watch was generally good, during this session the agreement between an H10 and the watch wasn't that great. Anecdotally, I think this is representative of the type (optical watch), not the model (Polar Vantage).
Also in this spinning test, one of a few, the concordance was again not that great. Rob doesn't go into why this test is this way compared to other similar tests, and again, his conclusion is that the tracking is pretty good.
My intuition is that the wrist-worn optical sensor (watch) is not as accurate as the forearm, or upper-arm worn optical sensor (Verity Sense), which is not as accurate as the EEG strap (H10). I'm not all that interested in getting even marginally bad data, and I already have a Fitbit to wear as a fashion accessory, so I think I'll be sticking with my Verity Sense, worn on the forearm, for the time being.
