Yes, they are enforceable. Traffic citations are, for the most part, civil violations not criminal. But you still must be properly served in order for the court to find you responsible. Mailing it to you is not service under the rules unless you voluntarily accept service by signing it and sending it back, acknowledging that you received it and that it is you in the photo..
If you do not do that, then the citing entity (city, county, state, etc) must serve you like any other law suit. And then it must prove that it was you in the picture, that the camera and radar were properly calibrated, etc. Theses are all easy things to establish, but the state still has burden of doing so.
Whatever your opinion of this process, it may be cheaper to acknowledge and pay the ticket, or go to defensive driving school if available to you, than paying the fine. The facts of your situation will dictate what is the best thing to do.