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Fitness Trackers Out of Step When Measuring Calories, Research Shows (theguardian.com)
Fitness devices can help monitor heart rate but are unreliable at keeping tabs on calories burned, research has revealed. From a report on The Guardian: Scientists put seven consumer devices through their paces, comparing their data with gold-standard laboratory measurements. "We were pleasantly surprised at how well the heart rate did -- under many circumstances for most of the devices, they actually did really quite well," said Euan Ashley, professor of cardiovascular medicine at Stanford University and co-author of the research. "At the same time we were unpleasantly surprised at how poor the calorie estimates were for the devices -- they were really all over the map." The team tested seven wrist-worn wearable devices -- the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn, and Samsung Gear S2 -- with 31 women and 29 men each wearing multiple devices at a time while using treadmills to walk or run, cycling on exercise bikes or simply sitting.
Apart from a power meter on a bicycle, you are probably right. It really hard to get a good estimate of calories burned without actually being able to measure how much work the person is doing. The heart rate is only very loosely correlated with how much work somebody is doing. Heart rate can be changed by a number of factors, many of which have little to do with how many calories you are burning. Power meters on bikes are a whole other story, because they can actually measure how much physical work you are doing, and will be able to measure calories burned within some reasonable level of accuracy. Even, then, you probably need to develop a base line, as each person will expend a varying amount of energy to produce the same output.
Anthropic principle: We see the universe the way it is because if it were different we would not be here to see it.
From the blurb, "...errors on energy expenditure... ...ranging from the lowest at 27.4% for the FitBit Surge to the highest error of 92.6% for the PulseOn device."
There are wearable gas analyzes but you will look like wearing a gas mask. But those are medical and accurate
For some models, yes, but by and large your information is dated. There are plenty of "consumer" models that are less than $500, and provide data that is quite good.
For more information than you can shake a bicycle crank at, I suggest taking a look at DC Rainmaker. He owns every device he reviews* and does lots of hands-on testing. For reviews of power meters, he typically tricks out his bike with 2-4 power meters (in different locations: crank arm, spider, rear hub, etc.), each with their own head unit.
* As he explains on his website, he often is doing his hands-on reviewing with pre-production models from the company, but always returns is and purchases his own - to confirm that is wasn't a special snowflake - before posting.