Actually, it is not his fault he is so mediocre in attack. Players like him suffer the most when there is no functioning attacking system in positional play. If we had that, we at least would possess several trained ways of creating a chance, like movement-and-passing drills, so that Semedo for sure knew where to move and what to do. Just do what he is told. But there on the pitch with EV in charge, when we rely on individual brilliance solely, Semedo rarely has a clue what to do with the ball.
Yea, he's very much a Kyle Walker type. Very athletic and fast, can do several thinks, but he is a very basic player in terms of vision. The system needs to create plays, not him individually.
Difference between Roberto and Semedo is easy to explain.
Roberto has mediocre basic abilities for the RB position (not very fast, not strong, not very good 1 vs 1). But good secondary abilities thanks to the fact that his vision is better (he is a midfielder after all). Semedo is the opposite. He has the core skills for a RB (fast, strong, hard to beat in 1 vs 1 and a fair dribbler), but his vision is not great.
As I said, he is a Kyle Walker type. Brains isn't his thing, but has good skills that can be put to good use by a top manager who has a drilled system in place.
Another similar player to Semedo is United's new RB, Wan Bissaka. These are players that do their basic job at RB well, but struggle to bring their individual touch on top of that.