You must be registered for see images
Yes, Hanzo is very underrated.
This is a man that beat the Sannin, a team of at least Kakashi-level shinobi whom were famed not for their individual power, but for their flawless teamwork. And they in turn became famous for not winning against Hanzo, but surviving, and his words weighed so heavily that everyone regarded them as the Sannin ever since.
He embarrassed Mifune, the leader of the samurai, and once had the power to defeat Pein.
People are only considering Hanzo as he was shown in the war against the Shogun Mifune, and he was weakened and never got a chance to use any jutsus.
So basically until Mifune fights someone else that has feats and is impressive, everyone will remain to look down on Hanzo.
He got one shotted by pain and soloed by a fodder.
It's views like this which hold Hanzo down...
Hanzo was old and weaker when Pein killed him, in his pajamas in his home none the less.
Pein said that Hanzo wasn't as powerful as he used to be, Hanzo lost his convictions and his skills waned from years of isolation. Pein also said that Hanzo used to be powerful enough to have defeated him.
When Hanzo was resurrected, he was resurrected in his weakened state. Mifune also commented that Hanzo was not as powerful as the man that whooped him all those years ago, noticing that he lost his edge.