I'm a lot more concerned about him barely being able to stand on his left foot, let alone pass with it. It renders him ridiculously predictable at times. People say Becks has/had a shite left foot, but he could cross and shoot with it, and sometimes to wonderful effect.
Nani crosses better than Valencia, even with his left, when he can be arsed. We saw that LOADS in his first season, where he often used his acceleration and quick feet to get the space to get to the byline for the cross with the left, and then he'd cross an absolute peach (considering it's his weak foot) nearly every time. What's happened with Nani, I don't know, but for the time being I'll not be too critical of Valencia, as he's clearly our best winger atm.