The tests do not need to be administered "perfectly." In fact, there is no "perfect" way to administer them. Heck, *I* don't administer them "perfectly," but, I administer them appropriately.
One has to recall the main purpose of these tests when they concern DUI - they are designed to test one's ability to perform two tasks at once. That is the important thing to observe in the tests - can they follow two directives at one time. Since driving is a divided attention task, the tests are geared towards addressing that. The biggest problem that most impaired drivers have is following instructions, and, yes, THAT is one of the biggest "cues" that we see. The test is not just about walking in a straight line, waging a light in front of their eyes, or standing on one leg.
Additionally, there are tests that are administered that are not part of the NHTSA test battery and still lawfully part of the FSTs, and the tests commonly used to evaluate for drugged driving are in addition to the NHTSA battery yet have validation elsewhere.