Their use for slow moving vehicle should be outlawed in every state. In most cases, it overrides the vehicles turn signals. If you are along side a big rig, and he flips them on, it appears as if he is getting ready to turn into your lane. This may cause you to take evasive action and cause an accident. And as Dave said, it can also cause an issue with brake lights. If you are behind a truck that has the hazards on, and the truck hits the brakes, that extra second it takes for you to realize they are no longer blinking could also cause an accident.
A safer and more effective alternative for vehicles that routinely must drive much slower than the posted speed would be for them to require one of those amber rotating lights; the same kind that snowplows and utility trucks use. That way, you still know to take extra caution when you approach one, yet it does no affect the turn and brake lights, so you still know what they are doing or planning to do.
Kris