Regarding soles, many shoe lines have complimentary models where the road shoe shares the sole with the mountain shoe. The mountain version just has stuff attached to it, knobs and whatnot. My mtb and road shoes have what appear to be identical rigid soles, with the difference being the extra mtb stuff. Doing a "in my hands" flex test showed little perceptible difference. (Sidi models, with similar uppers, like the Genius road shoe and Dominator mtb shoe).
I did have "comfort" SPD shoes and those typically have a hard rubber sole instead of a thick plastic rigid sole. The comfort SPD shoes definitely feel more flexible, to the point where the arch of my foot would cramp.