I don't think anybody in the world underrates him offensively. Defensively, it is almost more of a philosophical question. In his own zone, in contained defensive situations, he is bad. He fails to cover his man, he is weak on the puck, he is caught cheating. But. Because of his extraordinary puck skills and offensive vision, the amount of time he is contained in his zone in purely defensive situations is limited. And he moves the puck out well without making many bad turnovers. I think it's a fair argument that since the skill he brings leads to less time spent in the defensive zone, therefore he's of net benefit to a team defensively. That is the whole "a good offense is the best defense" cliche again. But if the question of his defensive ability is focused only on how he behaves in his own zone when he doesn't have the puck, that's where people can rightly point out weaknesses in his game, or suggest that he's "bad defensively".