Sorry was not suggesting you were a novice, simply correcting the impression your post may have inadvertently left for others given your status here.
Correct that topo's in Nuvi's do not have the full features they enable in my Epix or Rino's, however a good topo with embedded DEM in a suitable Nuvi such as for example the 3597 or Street Pilot on Android does provide some extra functionality such as great 3D rendering when driving in hilly or mountainous terrain which greatly assists visual navigation. And the missing functionality generally is not applicable to the driving use case anyway. E.g. gradient of the hike route etc.
And many topos such as my local Aust & NZ Topo do provide excellent full routing, including down to the level of remote 4WD track networks. Apart from some potential misrouting due to breaks in the vector data (which also can occur in CN as the regional source data for roads could be the same) I've never the last 300,000 km of touring here experienced a dangerous functional error such as your roundabout example. Nor in my travels in the European Alps on the wrong side of the road for that matter but the sample distances have considerably shorter.
That's not to say there aren't risks as clearly there are equipment and regional/data differences but the point is it's not a clear "likely" to happen and as with most things due consideration should be applied.
Cheers
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