All tools do more damage than good without competent hands around them. (Imagine a crazy moron with a Skil Saw). Some materials are the same.
1. The wrong color. Color charts should be taken "with a grain of salt". This is appropriate size for this metaphorical grain:
NEVER INSTALL COLOR HARDENER BASED ON THE CHART WITHOUT MAKING A MOCKUP FIRST.
2. Under-application = wasted money. If there is not enough material on hand or if the finishers don't realize how much really needs to be broadcast into the slab, the color hardener will end up somewhere between the fines and the grey cream on the surface. That is, you will not end up with any color other than the natural grey, and the money you spent at the decorative concrete supply house will be completely wasted.
3. Lack of education + color hardener = bad finishing practices. If the concrete finishers are not ACI certified or have no clue about the chemistry of concrete, odds are they are doing things that make the concrete much weaker than it is engineered to be. Add color hardener to that and you will likely make it worse. You can tell a guy to let it wet out before floating it, and you can tell him not to overwork it or to apply it at 50sf/bag or whatever. But if he doesn't have the understanding of what is happening at the molecular level, he will not be as able to improvise as conditions change to create great concrete. If the concrete is going to be covered, you can get away with some bad practices. If it is to be a finished surface in a custom home, the guys had better know what you are doing.