They now have textured steel that mimics wood. Fireproof and Class IV impact resistance (which, around here, gets you a healthy 25% reduction in your premium).
Good, thick A-grade shakes are okay if you seal them and put down some fire retardant annually. If not, well, you might as well pour gas on the roof. As the shingles wear and dry they tend to fray along the grain. All those little fuzzies are a perfect place for stray embers to get the party started.
Brad