There's not much they can do if you refuse to give them access to your data. At least in the Ninth Circuit, and arguably everywhere, they need reasonable suspicion of criminal activity to then seize it and try to break the password. That's a pretty high burden, the officer will need approval from a supervisor or perhaps higher, and they are not going to seize your device and waste their geek squad's time trying to de-encrypt it out of spite, unless they have good reason to believe there really is something bad on there.