>>76289235 (OP)
I've reviewed a few studies testing these shits... There's a lot of variability on different watch brands. I'd suggest Garmin, Withings and Polar. Apple, Samsung, Honor etc. are good enough though.
>GPS tracking
Great if you like to ROON, enough accurate distance traveled / time.
>HR
Quite useful to track on workouts, most +100 € sport watches have relatively good heart rate tracking. Not that useful on lifting tho, but good in general to track resting HR and general HR during the day.
>Sleep tracking
Most high-end watches actually can track sleep very well and quality of it good enough, it gives valuable data of how much you're actually getting sleep.
>HRV
This is very useful for gym bros. Most watches don't have accurate HRV meter, but solid enough.
>Calories burned
Great as well, quite accurate.
>ECG/Sleep apnea
Some watched are surprisingly good at tracking these. Not clinically good, but they certainly give data which can be used clinically.