So ive used Fitbit for several years now and to me it always seems it shows a higher caloric expenditure than what is actually going on. In theory I should be dropping weight with how much its showing ive burnt each day, but I am not. So there is a lot of other variables going on there. But its absolutely a good tool we just have to realize that nothing will be a 100% accurate. I mean even if we measure and weigh everything we eat, the error marginals could be high because of variety in caloric content of food from what the labels tell us.
And just to be clear, i absolutely LOVE the math part. Sitting down and calculating on macros, micros, calories in, calories out etc etc is almost as fun as going to the gym for me. For those who have taken a look in my training log will know this haha.