Obviously he was not unbeatable since Pain beat him, however, it is true that Hanzo is one of the strongest shinobi in his era, manga pretty much says that. People thinks low of him, since he lost to Mifune, however, Mifune is rather strong, when using IAI, and it's not something you can easily fight on short range.
Actually it would appear that Hanzo is more of a long range fighter. His Salamander seems perfect for it, since it can hide underground. And his poisonous breath is useful, since it makes sure his enemies cannot get close to him, thus he avoids getting into close range fights - normally. So we don't really know his full potential from that silly fight.