Roberto has his leathal runs, while Semedo doesn't (yet). Simple as that.
Still, in away CL ties, I would play them both and bench Dembele.
It is not only about Roberto's solo runs.
He has our DNA, literally and knows how to play a build up and a possession game.
On the other hand, Paulinho, Dembele and Semedo are probably 3 players with the least "Barca's DNA" currently in our team.
The same as how Paulinho is unfit for our midfield standards, that way Dembele and Semedo are strange fits for our style of play.
They DO have some positive individual skills, though, way more than Paulinho (at least Dembele).
But, when you play Semedo and Dembele on the right, and if you put Paulinho or Roberto as a CM, you are losing Barca's possession style and we look like some counterattacking teams, like in Lucho's era.
Again, that doesn't mean that Dembele and guys can't provide some good stuff.
But still, their current style of play hasn't got too much with Barca and with understanding with current Barca's players.
Semedo had some very good games attacking wise for Barca, the understanding between him and his team mates needs to be better but I think this can come with time.
It's remarkable how bad performances by Roberto, who has a lot of experience playing with the team get overlooked and forgotten quickly, yet a single mistake done by Semedo can make people forget everything he did well.
People are talking about Semedo's good games all the time.
What was that, 3 matches in September, because since then he had 1 good game in the next 20 appearances.