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I don't know for UK, but in Swiss Alps the data is extremely scarce, which paths outright missing and Maps showing paths that can be closed or extremely dangerous.

Apple and Google are not guaranteed to be updated when government road data changes. They do their best and they have tons of money and user-reported data (like live GPS traces from users' devices) to do decent work, but there are many times that OSM map nerds do a better job. The real issue with OSM is in remote areas where not a lot of people are around on the ground to realize that it's broken or care: the first user in an area often has some volunteer work to do. But fortunately it's easy and you can do it. Any random person can make a better map of their area than Google, instead of having to beg them for it.

Official sources also may have outdated maps.

Especially when for example bridge was destroyed hours/days ago.