Well problem with Semedo is that he gets isolated on the right side quite often, because Messi never is there, and Roberto can create plays in Messis area.
Im not against Semedo play Rb, i actually prefer him and rate him highly, but I also am aware why Roberto offers something differently, which can be usefull. The width Semedo creates if often wasted since he has limited options, but his athletism and defensive play still trumphs Robertos by a margin.