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.
Also watch out for "sugar by another name" ... pineapple puree, white grape juice/concentrate, apple juice/concentrate are very common commercial dressing ingredients to load up on sugar.
Sure always ask for the vinaigrette eating out, but at home make your own salad dressing:
* get a mixing bowl big enough to toss salad in, and a whisk
* add 1T dijon mustard, 1T not-balsamic vinegar (balsamic is high sugar! I like sherry or beer vinegar), salt & pepper
* drizzle in 1T olive oil while rapidly whisking.
* Add 3 oz or more salad, toss, done for 2 servings
There is actually an elegant _mathematical_ solution to this problem using sensor fusion and a differential equation model of the science: if you weigh your food almost all of the time at home, and only make portion and ingredient guesses when infrequently eating out, we can actually estimate your personal rate of underestimation and correct for it.
Our startup (BODYSIM.com) has also been doing research on this a long time. As founders, we all have >16 months of daily food logging mostly by kitchen scale weights, aligned to daily BIA-scale weigh-ins, fitness tracker calories, bi-weekly blood tests, monthly DEXAs, 3D scans, etc etc. We also have a science-based structural model of macronutrient balance and muscle hypertrophy. Given all that, we can VERY confidently estimate your TDEE (total daily energy expenditure) and its components, and predict how your fat and muscle mass will change on a daily basis. This is real math/science so you can also run it in reverse. This ("simultaneity constraint") provides enough constraints we can estimate users' individual underestimating/over-indulgence when eating out. In fact, it's better to just NOT log those days AT ALL and we can fill them in. I think this solution isn't more widely used b/c you need all this other "quantified self" type data at the same time.