From a first gut feeling I agree with the premise of the question. Also I sometimes feel like “extra curvy” prefers roads without speed limits set at all. But that could be selection bias cause it tends to find more rural roads, where the OSM data may not be perfect.
In Switzerland I felt like the “curvy” algorithm makes quite reasonable choices and leaves the cities quite soon.
In Poland we went through a shitton of 50 roads, but that might be because of the country’s road characteristics itself.
I will bookmark this thread and try to report some 30 examples if I find some!