Take a look at the DCrainmaker article here. He goes into detail on all aspects of the Whoop including HR accuracy.
Summary of HR:
"Of course, you’re likely realizing what the pattern here is by now: Relatively stable workouts Whoop tracks well. But anything with any significant intensity, and Whoop often will lose the plot.
And that’s the real issue here: Once you add short high-intensity, the accuracy of Whoop tends to go down substantially. Longer intervals tend to be better. This is a pattern that’s not at all uncommon. Many watches’ optical HR sensors share the same issues, though, I feel in general Whoop’s sensor is more prone to high-intensity errors than others.
But that ignores the biggest difference: For most other sport watch/wearables, you can pair to another heart rate sensor (like a chest strap) and get more accurate HR data if you need it. You can’t with Whoop. So effectively it becomes bad data in, bad analysis out."
My own experience is that it is much better on a bicep band than on my wrist when it comes to measuring HR while riding, especially mountain biking. There are times it works really well but other times it can be wildly off. My Garmin Fenix 5 watch is similar - sometimes works great for HR on the wrist but othertimes way off. I’ve gone back to using on Tickr HR chest strap for most rides just because it is so reliable.