André Gomes and Paco Alcacer also joined Barcelona from Valencia, both very highly rated at the time, and both have been complete shit. Cancelo's alternated between RB and RW during his time at Valencia precisely to put less defensive pressure on him because he's so good going forward but very, very poor defensively.

Watching both Nélson Semedo and Valencia every week, I think Valencia is better defensively than Nélson Semedo. Nélson still has some strange decisions in him when the team is defending, especially in positioning, whereas Valencia has really improved in that regard over the years, and, like you said, Valencia being physically stronger and his use of that physical strength helps. Offensively, however, Nélson Semedo takes the cake. His crossing is better, his dribbling is better, gets past players more easily. It's this type of attacking potential of his that allows Benfica's right wingers, Salvio and sometimes Rafa, to drift into the middle and make runs to open the opposition defense up, these players know they can count on Nélson's offensive capabilities on that right side when drift to the center. In our case, players like Mkhitaryan, Mata, and Lingard also perform those runs and drift into the center, but I don't trust Valencia on his own offensively anymore. In those situations, I think Semedo would be a big, big improvement on the team's offensive potential.