Unbend only unbends solid cylindrical sections with coaxial edges, and it doesn't even recognise the offset face of a shelled section (the back side of the bend area). Unfold Face is more liberal, dealing with conical sections (of which unbend sections are a subset, but Unbend needs a solid, while Unfold Face only needs, well, a face) with no restrictions on trim edges, but neither deal with ruled surfaces generally. Ruled surfaces are developable single-curvature surfaces, but there's no way to constrain F2F lofting to a general ruled surface, let alone a ruled surface that's a guaranteed conical or cylindrical section.