As I said in my edit summaries, consistency between articles matters. That's how Wikipedia works and lists are not an exception to this rule. Even if several references call System of a Down a "nu metal" band it does not mean it should be included in the list. Other sources clearly say this band is not a nu metal (see here).
Also several sources used in this list are not reliable, and I really don't see what makes this first source, this second source or this third source (a clearly Googled search which doesn't even say the band 311 is a nu metal band) reliable. This list really needs some clean up anyway.