There are technical and legal defenses available.
Someone charged with drunk driving might be able to raise some technical defenses to the charge. For example, that the officer didn't administer the breathalyzer test within two hours of being pulled over, or that the equipment wasn't functioning properly, or perhaps that the officer didn't administer the field sobriety tests appropriately. In my experience, often, the most effective method to challenge such a charge is to challenge the officer's original reason for pulling the motorist over onto the side of the road. If that officer did not have a reasonable suspicion that criminal activity was afoot, everything that comes after the stop is inadmissible at trial.