They are not supposed to be an accurate method to estimate calories burned. Just a way of motivating yourself to do more walking.
The resting hr feature has some use ie suggesting you are not right. This could be overtraining, hangover, minor illness ora variety of other things. I could just take my pulse but rarely bothered. Writing the rate down in a way I can easily correlate it with activity was even less likely to happen.
The sleep tracking thing made me think about how I feel with varying amounts of sleep.
I still use my charge hr and think it was worth buying.