Not great! We have to "correct" them mathematically. Some can be crazy off until you calibrate them, for instance my husband's then-new Apple Watch was 2X off for months until he transitioned from winter indoor treadmill running to an outdoor run where it can calibrate. But even Garmin/Whoop/Oura can be substantially off, 50% easy. A "fun" simple way you can test yourself: do the exact same weight lifting workout but at + or - 10ºF between the two replicates. Guarantee you'll get dramatically different "calories burned" when in reality they are more similar. Your heart beating faster to cool you off in the hotter environment even though you're doing the same amount of (physics definition) "work" is NOT burning 50% more calories. Heart Rate is just a proxy for "work" and is confounded with other causes of heart rate change, such as ambient temperature/cooling.