I once mapped out a route from Peterborough to St Neots (weekend block on the ECML south of Peterborough!) on Google Maps - it was so bad it was almost funny. The route took in pretty much every cycle path in south Peterborough, winding around housing and industrial estates to travel about 15 miles for a straight line distance of about 5 miles, then immediately joined the A1 at the junction where the Peterborough bypass A1(M) finished - four lane motorway standard, 70mph and all - for the rest of the trip to St Neots!! That was when I switched to Strava for journey planning!Yes, I find them to be accurate for an "average" cyclist. A road bike user would probably be a bit faster. However there are better routing tools out there - Google Maps will sometimes put you on unsuitable dual carriageways or tortuous paths through housing estates. It also loves canal towpaths even when quiet streets go the same way.
If you use Strava then its "popularity" routing option is good plus it will estimate the ride time based on your own history.
True.Ultimately a good cycle route planner would account for your preference between "dual carriageway if it does the job" and "the quietest % shared with motorised traffic at all costs"