Heh heh. I've been avoiding hill running while I get some hamstrings under control but tonight I ran my 4km "home block" barely faster than a good walk. Garmin feels that was "highly impacting". In the first part, downhill and flat, it claims HR was 140-145. And cadence was guess what 140-145. At the start of the sustained uphill, HR mysteriously dropped 20bpm, then gradually rose again. Well that part might be true. But I doubt it. I was completely un-puffed.
To have any faith in the Garmin wrist HR, I have to wear the watch facing inwards toward the body and uncomfortably tight.
And then, perhaps, the GPS aerial is shielded by your body and the positional accuracy is worse. An example of "the invisible arm of the market".
GPS isn't that bad. Its one of the older models so the antenna isn't micro. I regularly have this on together with an eTrex and the Samsung inbuilt tablet gps when mapping. If it wasn't for the "smart recording" function, I think the forerunner would do better than the eTrex.