I found it helpful to think of this subject in 2 ways;
1. There is route planning and as
@YukonBoy points out most devices are not very good at this.
2. Route following - whether by gps device or phone or cue notes.
Route planning on the fly normally requires internet access and/or subscriptions or previous paid downloads. Routes are typically not ideal tending to be focused on destination, not journey.
I've tested various online planners and prefer cycle.travel as it chooses quieter routes. I found both Komoot and RideWithGPS to be more direct, although the advantage of RWGPS is to "borrow" other's routes. To test, I used them to plot routes between places known to me and then checked which one suited me best.
A simple and cheap way of getting in to the whole thing is to try Osmand on your phone, coupled with your preferred planner. Create route online, transfer gps file to phone, load gpx in Osmand and off you go.
There are many discussions on this topic arguing the merits of different planners and devices. Have a search.
I always advise anyone to have a real good think about what they want before buying a device. Each device has advantages over another and disadvantages too.