Stichodactyla haddoni isn't always necessarily impossible to keep, but the more fantastically coloured ones might be. In the last year or two I have seen so many blue saddle carpets popping up all over the place whereas before it was unheard of. Where did this sudden cache of blue carpets get found? I hate to be a cynic but how do we know that some of them might not be dyed. When they're dyed, it takes an animal already on shaky ground and makes its chances for surviving that much slimmer.
Like any anemone, a happy and well-adjusted carpet can be the easiest animal to care for. A stressed out anemone is a miserable thing and can easily succumb (i.e., die). Guess what? All anemones are stressed out after being collected frpm the wild, shipped, and subjected to the retail process. Thus many, many don't make it through the first year. It's a very sad phenomemon. It's a horrid statistic. DON'T buy an anemone unless you absolutely for sure know what you're doing!
Carpets, in general, can be problematic for those unaccustomed to them. For one, they get big. Secondly, and certainly not least, they eat fish. (At least S. haddoni does...)