The problems lies in proving the driver was impaired, and sufficiently impaired to be guilty of violating the law. That is where I would expect to see charges attacked by defense counsel.
The rule regarding what constitutes impairment varies from state to state. The OP is evidently in NC. The rule in that state, which is quite longstanding, is this:
The correct test within the meaning of the statute is not whether the party charged with the violation thereof had drunk or consumed a spoonful or a quart of intoxicating beverage, but whether a person is under the influence of an intoxicating liquor or narcotic drug by reason of his having drunk a sufficient quantity of an intoxicating beverage or taken a sufficient amount of narcotic drugs, to cause him to lose normal control of his bodily or mental faculties, or both, to such an extent that there is an appreciable impairment of either or both of these faculties.
State v. Ellis, 261 N.C. 606, 607, 135 S.E.2d 584, 585 (1964).
Compare that with Colorado, which has two offenses, driving under the influence (DUI) and driving while ability impaired (DWAI). DUI is the more severe offense, and requires that the state prove that the driver was affected “to a degree that the person is substantially incapable, either mentally or physically, or both mentally and physically, to exercise clear judgment, sufficient physical control, or due care in the safe operation of a vehicle.” CRS § 42-4-1301(1)(f). For DWAI the state need prove only that the driver was affected “to the slightest degree so that the person is less able than the person ordinarily would have been, either mentally or physically, or both mentally and physically, to exercise clear judgment, sufficient physical control, or due care in the safe operation of a vehicle.” CRS § 42-4-1301(1)(g). So in Colorado, if there was impairment to even the slightest degree because of alcohol or drugs the driver is guilty of at least the DWAI offense.
Point is that the law varies from state to state and in some states, like Colorado, any sign of impairment combined with proof of having used alcohol or drugs can be enough to get you convicted.