UnrollSrf is for surfaces curved in one direction, i.e. developable. Smash will flatten a surface that has curvature in two directions but this implies some expansion or contraction in areas so it’s not as exact. Here’s some further info on the help pages for these commands.
http://docs.mcneel.com/rhino/5/help/en-us/commands/unrollsrf.htm
http://docs.mcneel.com/rhino/5/help/en-us/commands/smash.htm