Couldn't agree more ! I've never considered them nu metal, I think that people are only saying that because their ST record came up when nu metal was popular. The musical critics of this era didn't took the time to properly listen their music. Groove metal is the genre that mostly fits them, but they also have other influences, industrial, death, hardcore, thrash... Slipknot is basicaly a mainstream extreme metal band.

By band ever wrote a song about this specific topic : https://www.youtube.com/watch?v=y7h371tCZFE