Question about Garmin Edge 820

Short answer: INADEQUATE!
Long answer:
- The display is too small.
- The touch screen is an imposition, unusable in the rain, unusable with gloves. No matter which setting.
- The pre-installed map is not only extremely slow on the device (zoom in and out approx. 5 sec per step), trails and forest paths are barely recognisable. Thin black line on dark green (forest) background. Zero contrast. It works with an Open Streetmap.

I previously had an 800 and often only decided where to go on the bike with the map view. With the 820, I hardly ever use the map view. I don't think it's possible to find a route while riding, you have to stop, take off your gloves and have good nerves. The best thing to do with the 820 is to plan the route at home and follow the track.

Yes, the device is absolutely suitable for biking in rough terrain and is also well protected with the silicone cover. Rain and mud can't do it much harm.
The ultimate are the maps from Swisstopo, which contain the single-trail maps. With a loaded track, however, it also rides very well without.