It [cycle.travel] often jumps onto a convoluted route along a National cycle route when I’d prefer a more direct B road. Plus it absolutely hates A roads even if I’m deliberately trying to plot along them, because I know it fine.
(Sorry, a long post)
I think all these online route creators have to base the routes they take on weighting different considerations for any particular road. My understanding is that in the UK cycle.travel includes traffic volumes as part of it's weighting system (along with other factors) and whilst these can provide an "indication" there are also constraints eg time of day as some roads might get very busy in rush hours but be fine eg mid-morning.
I have a "test route" (just start point and end point) that I put into online route creators and some, even some dedicated cycle route sites generate a route I can only describe as "certain death" (except when you get to the slip road you'd turn round and find alternatives rather than blindly head on) eg routing you down the A14 past Ipswich.
In my experience cycle.travel has always given me excellent safe routes (ie NOT along the A14 past Ipswich!) but as with all such systems based on algorithms what you find when you get there is the deciding factor. On occasions cycle.travel has done some trivial weird stuff like in UK getting you to turn off the road onto a parallel service road for 200 yds then back onto the road you were just on.
Probably the weirdest is Garmin created routes with "popularity routing" enabled. Took me to ignoring a few routes until I discovered how to disable the option. It can route you is significant detours to get you to a road for no reason other than Garmin has recorded a lot of cyclists using that route. Near me there is a horrible B road I don't cycle on but it is popular with peloton groups (20+ mostly carbon riders out training) and then think they probably all have Garmins, all "contributing" to popularity routing so it's no surprise Garmin isn't sending you down that beautiful scenic deserted country road. BUT you can disable it and return to more sane routing.
Additionally, all these systems are dependent on the data (normally OSM) and it isn't 100%. Most errors I've come across are minor eg road classification or gates, ,etc. and how those affect created routes will vary according to the weighting system the site is using.
Ian