Everything except that he's not left footed and there will be times Pep would want a left footer on the left. If you break down the 2 halves vs. Villa, Cancelo was higher up on the left attacking with Bernardo covering the space behind him. Cancelo was there as a psudo winger/wingback while Foden came narrow. However, Cancelo had a hard time creating from that position because of Villa's shape and how compact they were.
That's why Pep brought in Zinchenko in the 2nd half. He basically did the exact same thing Cancelo was trying, but had more success because he's left footed and could go around toward the byline easier. Pep even said this. So Cucurella makes sense IMO.