I have had a very low HR for all my life. When I was younger I ran 6 miles / day on a regular basis. (Can't do that any longer because of back and knee issues). I'm nearly 72. I walk on my treadmill 60 miles / month on a 10% elevation (as high as my machine will go).
My resting HR as measured by my Polar HR machine and my Dr's HR is normally about 40. Has been around 38 often and low as 36 occasionally.
My cardiologist thinks I'm doing well. His stress test will increase my HR to 130 but I'm pushing hard to get there.
I don't like the strap on my chest that the Polar system requires but I realize it provides the best opportunity for signal so the sensors provide an accurate report.
For me, accuracy is everything. Polar has it.