Another example: a lot of rural areas have patches of gravel roads. It's not obvious from Google maps where the roads go from paved to gravel (and they sometimes do it in random sections).
Naively looking at a map, you'd think a roundabout route had potential to be shorter, but not realize the road was gravel.
Surface data in OpenStreetMap is pretty incomplete, but it is easy to add to a road and is well used when present (cycle routing is a popular use of OSM and avoiding soft and uneven surfaces is handy for cycle routing).
In rural areas, school buses drive on gravel. Where I grew up, the buses were speed limited by law, so the speed on gravel is often the same as on the highway. Road surface might not matter as much as you think.
In rural areas there are a lot of minimum maintenance roads. Legally they are roads and you can drive on them all you want. However if you don't have 4 wheel drive you shouldn't drive on them, even with 4 wheel drive there are times when you won't make it. A bus should never drive on these roads, and google does not know which these roads are - as I've discovered when I had to rescue someone who blindly followed google maps.
Sure, I guess I was trying to comment specifically on gravel. You will need accurate maps for this to work, but I think that is basically a given. A human route planner will also need to have accurate maps.
Naively looking at a map, you'd think a roundabout route had potential to be shorter, but not realize the road was gravel.