For anyone else experiencing something similar, try the 'Avoid potentially unsafe roads' option in the settings menu. This tells the routing algorithm to always avoid 'secondary' and 'tertiary' roads without explicit sidewalks (these are OpenStreetMap road classifications).

This isn't on by default because in some countries/regions, such roads are very reasonable to run on.

Clearly there's a better middle ground though, which might involve highlighting potentially problematic parts of the route in the UI.

Thanks again js2 for your email and all your feedback!