I've grumbled gently about cycle.travel on another thread, and @chrisfl seemed interested so it seemed wise to move my observations into a new thread of its own.
I should start by saying that cycle.travel is my route-finding website of choice. I find it particularly good for plotting long routes, as it's the best I've found at knitting together NCN routes. It's also good for plotting short routes in town: it generally comes up with the route I'd pick, and on that basis I recommend it to others. I recognise that it's free, and that it works (most of the time).
However, it has let me down a couple of times recently, by making slightly perverse routing choices. Sometimes I suspect that's because the basemap is wrong. Sometimes I suspect it's the way it does routing.
Here's an example of both at once. See how the route diverges from the A701: cycle.travel does this quite a lot, it seems terrified of major roads, even when it's "obvious" that if you're on the A701 at the start and end you might as well stay on it. However the second problem with the diversion off the A701 here is that it goes up what is clearly a private drive, and then through two farmyards.
Similarly near Coldstream it's routed me through the private grounds of an estate, rather than follow the NCN around the edge.
...and near Longtown it routed me along an old railway on NCN7 that was muddy single track despite me choosing 'sealed only' as a routing option.
I still reckon it's a good tool, and I prefer it to Strava or Garmin Connect for long distances, but it can be a bit dense sometimes.