I think they're more simple than not. They have lots of unisons, with the bass and guitars, and also with the vocals. Their songs typically have a main riff which they repeat over and over, then they'll go into a verse where the guitars get simpler -- either rhythm on one chord or a slow progression -- and then it's back to the main riff. I think that's pretty simple. A unison bass line, to me, is far from complex, since you can't get much simpler than a unison. They don't really get harmonically complex. They either have the guitars doing an ostinato under the vocals, or the guitars follow the vocals with power chords. To me, the odd vocal melodies seem to be more embellishment than compositional complexity. The one area where I think they could be said to be complex is their rhythm. Although neither of my reference songs change time, they're both rhythmically interesting. I'll give you that. Still, that's just one aspect of the song.