Here is one of my runs in the forest. If you zoom in a bit then you´ll see how bad it is. I had the same result with my Fenix 3 but its much better with 305 and 601. So I guess the GPS in Fenix isn´t very good under these conditions.
https://connect.garmin.com/modern/activity/2368390982
Not sure if you bothered to look at my post, but if you zoom in you can see that there are points where all 3 devices strayed at different points on the route. My point is that you are not going to get much better result from any commercial grade GPS device that is small enough to fit in a watch. Sure some chip sets are slightly better than others, but they are all within an acceptable range for commercial grade devices.
I own the fenix 5x, Polar V800, and Suunto Ambit 3 Peak. They are all usually within a few hundredths of a mile on the courses i use (3 to 10 miles), and they have all strayed off at times.
The fenix 5x has a definite advantage over other devices as it has the ability to view the map right on the device. For example if i put a course on my 5x, and i want to explore new trails during the course, i can see those trails right on the map and be able to know where i am and see where all the trails lead. You cannot do that with other devices.
JMO