I imagine no one who has watched Sano at third for more than a couple of games doubts he has the tools to play the position. I believe the caveats you hear are along two axes: 1) he's not yet consistent, and 2) he's one self-satisfied off-season from eating himself out of any skill position, so why invest further time developing him there. There's also the presence of Plouffe on the roster. Plouffe is less gifted at this point in their respective careers, but more steady. Suspect defense, maybe. But I don't think the judgement we've heard has been hopeless defense.