Here is a comparison I did today with my Fenix 5 and Ambit 3 Peak.
Wore both devices on my left wrist. GPS set to best/GPS+GLONASS.
Both set to 1 second recording.
F5 OHR, and A3P chest strap
F5
https://connect.garmin.com/modern/activity/1688796912
A3P
http://www.movescount.com/moves/move152639386
MyGPSFiles tracks
http://www.mygpsfiles.com/app/#PhH2GGhH
Looks pretty darn good to me.