I have no idea how much credence to give the horror stories, but I have read warnings about frying the electronics in more modern cameras with older style flashes, specifically the Vivitar 285, because of high voltages across the hot shoe contacts. This is a shame, because that 285 is a great solution that I've used for years with a variety of cameras. I recently had a cord made that will connect it to my M3, but I'm waiting for more information before risking my M9. At this point, I might even be shy about using my Nikon flash on the M9.
If anyone reading this thread has accurate information, I'd love to hear about it.