True, but they also have Paranoid, which is far better than anything Metallica put out.
If you ask any metal band who is more influential, Black Sabbath or Metallica, a larger majority will say Sabbath. Every member of that band created something for metal, hell they CREATED metal. Every documentary on metal says that Sabbath was the first metal band. Without Sabbath, there would be no Metallica, Priest, Maiden, Pantera, Evile, Megadeth, Motorhead. Sabbath is far from underrated. Every metalhead knows that Sabbath started it all, so every metalhead gives (and should give) the same amount of worship to Sabbath as they do their favorite band.