My displayed heart rate is pretty accurate when sitting around and my resting HR seems pretty accurate also. When I climb a set of stairs it shoots up way too much (compared with an actual pulse measurement) and then coasts down. But when "walking activity" is used on a treadmill it starts out OK but then drops to around 100 for about 15 to 20 minutes, and then suddenly seems to jump and sync with my actual rate of 130 or so as shown by chest strap on treadmill and/or actual pulse. It seems that it is actually being "held" at around 100 for 15 or 20 minutes.
When cross country skiiing using "other activity" it just seems to stay around 100 all the time when in reality a pulse check shows 120 or 130 (I am elderly and that is my target). It is very frustrating as the inaccurate low heart rate is useless for helping me pace myself at a safe heart rate. I can guess much more accurately than the watch displays.
Is the displayed heart rate somehow adjusted based on the activity. The folks who review these devices seem to get more accurate results. They are younger and work at a higher level and often seem to be in the bicycling mode.