Hello,
Andy from OSM's Data Working Group here.  Whenever there's a question about something like highway classification I'd definitely suggest discussing it on the forum https://community.openstreetmap.org/c/communities/ca/95 .  It's true that there can be inconsistencies, but it's rare that there are wholesale category errors.  I'd definitely suggest asking to find out why something like https://map.atownsend.org.uk/maps/osm-deep-history/#/way/94692876 has happily be a highway=secondary since 2011.
Best Regards,
Andy
