If anything, Buffon has been underrated outside of Italy and Serie A fans for a large part of his career. For years most people would say that Kahn, Casillas, Van der Saar, Cech, Julio Cesar and others were the best in the world, when Buffon was by far the most reliable keeper since the late 1990s, when he was incredibly young. All of those folks (and others, like Dida, Cordoba, etc) had great seasons, but Buffon has always been by far the best. Real worldwide recognition only arrived after 2006.
And arguably since around 2010 there have been proper challengers, like Neuer or Courtois, but Buffon has done remarkably well to remain an elite nearing 40, although he is certainly far from alone in that.