Depends on who the "they" is.
If "they" is the testmakers, they never tell you the cleverest, most intuitive approaches to math problems; the Official Guides always lay out the longest, clunkiest, most grind-it-out solution methods. They also don't tell you specifically what skills are being rewarded in the verbal sections.
If "they" is professional test preparation companies, many (not all) don't tell you that the GRE isn't out to "get you." Graduate schools have no interest in screwing applicants, so why would they countenance a test that screwed examinees? Doesn't make sense.
If "they" is your peers, there's too much misinformation or disinformation out there to list here. But since you ask about sins of omission, probably the most important thing others fail to point out -- often because they don't realize it themselves -- is that successful test takers are not born, they're made. By getting the right coaching and by applying the proper habits and attitude, any examinee has a shot at the shiniest of brass rings, even one who has never been a fine test taker in the past.
