Reading from "www.nadex.com" - the copy reads "Binaries are similar to traditional options but with one key difference: their final settlement value will be 0 or 100. This means your maximum risk and reward are always known and capped.".
Isn't that true when you are using traditional options? (Assuming the markets are the same.)
Addition: Can't you essentially replicate the payoff of a binary option using a vertical ATM option spread?