I don't think Exodia is a bad deck, quite the opposite. I think the deck is too good.
On turn 1, before I've done anything at all, my opponent managed to draw through about 35 cards of his deck to draw all 5 pieces of Exodia and succeeded.
Not only is this no fun whatsoever, I think that's completely against what Konami wants from the game and should be banned. If I had to guess I think the only reason it hasn't been is because not many people play it in real life.