Assuming you're talking about the UK, the National Cycle Network is notoriously awful and full of mudbog footpaths, flights of stairs, soft sand beaches (!) etc because Sustrans favours "traffic free" over any sense of sanity.
https://twitter.com/hashtag/Sustransed?src=hashtag_click
So something being part of the NCN doesn't tell you anything useful about whether it's good to cycle on if you're trying to make progress.
Having had this job my memory of taking route one up the coast, you make a very good point!
@Nef started
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Assuming you're talking about the UK, the National Cycle Network is notoriously awful and full of mudbog footpaths, flights of stairs, soft sand beaches (!) etc because Sustrans favours "traffic free" over any sense of sanity.
https://twitter.com/hashtag/Sustransed?src=hashtag_click
So something being part of the NCN doesn't tell you anything useful about whether it's good to cycle on if you're trying to make progress.