Had been using a Fitbit Inspire HR for over a year. Intent was to have it for hiking/backpacking and mountaineering, for realtime HR, as opposed to a cheststrap HR like I've been using.
The Fitbit isn't terrible, and does give a decent indication of HR, when it's below 100. As the heart rate rises well above 100, the Fitbit can;t keep up and loses accuracy rapidly.
In comparison to my chest strap HR, a confirmed 140 would show as 110-112 on the Fitbit - so unusable.
I updated the chest strap to a Wahoo Kickr and using with my cell (which I normally don;t bring into the back or high country here in Ca - 95% of places have no cell signal).
Wife has a Fitbit Charge 3, which she claims is ok - but then she never goes over 100, so can;t really confirm.
My Fitbit is hanging out in my drawer...
Ride On
Yuri