It's not that the chords themselves are complex, they're not. They're just (as with most metal songs) standard power chords. It's that the actual songs are complex as a whole, with these simple power chords all over the place, all up and down the neck of the guitar.
It's like the difference between a classical piece, where you'd have one or two complicated, or complex (6 string) chords versus a metal song where you have 8-10 simple power chords (2-3 string chords) at all different locations on the neck, played very quickly.
With the former, you're focused on precise finger placement for these complex chords, where the latter, the power chords all have the same basic chord "shape" with the fingers. You're just moving your hand up and down the neck to different positions more rapidly. It's more like a workout! lol