It's the Meaningless Miracle. So many people don't even remember him for anything else. So the prevailing groupthink is "Marcus Williams is a bad tackler."
And that will always be truth, despite every single metric says the opposite. Unless we see him visibly make a game changing tackle. Maybe a big one...and the problem is, he's not like Troy Polamalu or Sean Taylor. He doesn't make explosive tackles. He prevents deep shot and picks off bad throws in his range (which is a lot of the field).
But what he does is often hidden on the edges of the screen we can't ever see because unless a QB tests him, he's invisible out there.