What if someone breaks off the outer knob?

The electronic parts that contain the secret keys and the critical decision making process (e.g. whether to unlock or not) are inside the body of the cylinder, which is securely contained inside your door.

Should someone break off the knob by force, or use a hammer or other tools to try to violently open the lock, they still would not be able to enter the door.

It’s an act of vandalism and it will cause trouble, but it won’t compromise security.