I tend to find that metal often "vamps" rather than "progresses." instead of the song moving forward using chords and the tension of cadences, it just moves back and forth between two notes, usually the root (which is typically E, or whatever the lowest string is tuned to) and the 4th, 5th, and my favorite, the minor 2nd. It really works with any of 'em. Metal's very open in that way. Just ride those back and forth a measure each and pepper them power chords as punctuation. After that, I usually analyze it and revise till it's a riff I'd actually like if I heard it on a CD. Just my $.02. I'm certainly not an expert (yet!)
It depends on what kind of metal but yeah,
some bands tend to keep it very simple using
root and a note extra or few.
Bands like Dream Theater or Symphony X are far more complex chords wise tho.