They propel projectiles at high velocity, and thus you don't want to get hit by one or point them at windows or other important things such as cats. And when you shoot things like ice or wood as opposed to potatoes, they have much more mass and become even more dangerous, considering they do not vaporise on contact. Combustion cannons use a flammeable propellant and thus have a danger of exploding or fire, however this is rare if proper methods are used. Combustion cannons can send the endcap of the pipe shooting off, and they do shoot large flames from the barrel. But they are generally safe when used with precaution and common sense.
Legality is a sketchy issue.
For all intents and purposes, they are a recreational device which shoots small projectiles at stationary targets. Obviosuly you don't want to go shooting billiards at billboards, but for the most part there shouldn't be a problem. Combustion cannons may cause a noise problem however. I advise you check with your state and local law enforcement agency for further information. The following is a letter that finds its way onto most spudgun sites about legality.