Theres nothing wrong with DDR2-800 RAM (if thats what you mean).
Although I think I may know why they are saying that. When you have faster RAM, it requires either more voltage, looser timings, or both, so with DDR2-800 RAM vs say, DDR2-667, a lot of companies end up raising the voltage of their RAM anywhere from 2.0v-2.2v. And with a few motherboard types, they do not work well with RAM that is clocked over 1.8v-1.9v, and in some cases the computer simply doesnt boot.