In fairness, I think a
complete failure of the device is unlikely.
In the event of that, and no phone reception there are two options suggested in this thread.
I did that for years, in various places. Heading in the general direction was enough for me. I put on my "explore" head and off I go. However, I have a go-anywhere bike so I'm not bothered about surfaces etc.
For me, this is one of the killer differences between Wahoo and most Garmins.
The flip side is that going off course on a Garmin Touring unit (to be fair a good few years ago) was a nightmare for the opposite reason - a small, colour screen with far too much detail and colour.
I want my unit to show me where to go and to record my ride. I don't need detailed maps. My phone is better for those.
There's a lot to be said for a good, big, paper map.
But this is the Touring and Adventure forum and carrying all the maps necessary for a long tour can be ......... (insert favourite adjective here
)
In fairness to Wahoo and their units I've used first an Elemnt, then a Roam (because a Bolt was unavailable) and can count on one hand any serious issues.
The biggest issue can be connectivity, and like you, I suspect that our phones can play a part in that.
I rarely use the app.
A word of warning on this, especially if in out of the way places. A route created in one planner may be "adjusted" on importation to another if the underlying maps or algorithims are different. To be honest, I don't quite understand the nuts and bolts of it, but I know it can happen.
It's perhaps best demonstrated on RWGPS where a route can "jump around" depending on the map displayed.