Wow, since you said shy dogs are dangerous and unpredictable, I suggest you stay away from shy dogs expecially your local shelter. Pit bull, rotweiler, and German Shepherd are the top three aggressive dogs. You said they are dangerous. You better to stay away from them too. If you can recognize the sign of behaviors a head of time, you can prevent dog from biting. There are no such thing as unpredictable behaviors. Your client's dog is different from your dog that you grew up with. You don't know the history and what temperament of a client's dog. Once a dog bite you, that is your and your client's issue and a potential of a lawsuit in the future.