Wood shingles of any kind are much more flammable than any other common roofing material. A cigarette lighter and a sample of the materials will very quickly demonstrate the difference. We've all probably set fireworks off in the middle of an asphalt street, but I doubt you've ever seen the street catch on fire. Nobody in their right mind would set off fireworks on their wood shake roof. I've only seen that once in my life, by a bunch of loadies who rented an old farmhouse to party in. They managed to burn it to the ground, along with a delapidated old barn, in the course of an hour or so. They promptly moved out of state.
Some areas have ordinances controlling their use almost entirely based on this very problem. Although you can catch asphalt shingles on fire if say, a bottle rocket lands on them early in its flight and continues to burn long enough, but it's rare and the fires are easily put out. It take very high temperatures to keep the shingles burning (look at the temperatures required to melt asphalt in a "hot tar" kettle). This isn't true of wood shingles if they are dry or allowed to rot.
They make a beautiful roof, but some common sense and reasonable maintanence are needed. Trying them out on a shed is, indeed, an excellent idea. I personally wouldn't use them on any building that's heated with a wood burning stove, but I think that's not unreasonable if care is taken to minimize any chance of sparks leaving the chimney and landing on the roof while they're still lit up. I just don't think I could sleep well at night, myself, worrying about losing all those chainsaws I left sitting out on the dining table.
