Right back is simply Barcelona's weak spot. Neither Roberto or Semedo is terrible there. I think Roberto works with teammates better (not surprising for how long he has been with the club) but not the 1v1 defender.
I think La Liga matches should be split down the middle, if Semedo's team shows a lot more fortitude, than play him in the CL, but it is on Semedo to show the team (practice, which we don't see, and matches, which we do) plays better with him.
To have both Roberto and Semedo as options isn't great, but not terrible either.
I will say I think Roberto is better as an RB than as a midfielder. I think he is top 15 or so in the world at RB, much lower than as a DCM, CM. The drop off from Busi, Rakitic or Vidal (probably willing to include Arthur too) in the middle of the park is pretty big, where as I think it is a little unclear the net benefit whether him or Semedo play at RB.