TimmyR,
The usual cause of high wrist HR readings is poor positioning and placement. The watch band should be snug, and positioned on the fleshy part of the wrist, well away from the wrist bone (this is higher and tighter than most people are used to wearing a watch). This gives the watch more blood supply to measure, and prevents external light from entering the sensor with each arm swing.
Helios,
The major issue is that of sensor connectivity can be problematic with some devices. Somehow the metal case of the F5 interferes with the antenna signal, and you can get sensor dropouts. Stages power meter is notorious for this, and Stryd has been problematic as well. It is worth noting that these are typically located a little further away from the wrist than say, a chest strap HRM. If it affects you, try transmitting on BLE rather than ANT+ (or vice versa).
Since Garmin had to redesign the the 5 plus to get sensors and GPS to work correctly, isn't it time for Garmin to acknowledge the fenix 5(s) doesn't work as advertised and provide an exchange program?
So I am still considering this watch. Does every single device gas GPS drop issue, is that general known issue ? Any happy customers around here?