It's pretty damn simple: He's not a RB. Having some good games every here and there doesn't equate to being able to just convert and play there long term. Most players can have a good game or two (Or three!) at a position they're unfamiliar with but as time goes on and all the smaller nuances of the position become more consistently an issue it starts to become clear they're not cut out for the position. Plenty of outfield players can dive for a ball and catch a bullet when playing goalkeeper and if they step in goal they can do a decent job of that, I'm a referee and I see it all the time, but these players will most often turn out to be really shitty goalkeepers in the long run because they don't have a lot of the smaller basics down that they simply wouldn't know due to not being familiar with the position. Even small things like when to watch the ball and when to watch the leg are entirely different when you're in your box or making a run towards it, and when you're in midfield. These things can be learned with a lot of time and effort but playing against the likes of Real Madrid and PSG are hardly the times to be learning them, as Di Maria and Gareth Bale probably don't care too much that the fullback marking them doesn't fully understand their positioning.
It was foolish to think Roberto could be the savior RB we always needed and all those who opposed signing a RB because they felt Roberto had the position secured and he deserved it somehow probably feel a little let down by a player shoehorned into a position unlike any he has played before.