I don't think they're all that difficult (as long as you feed them like fishfreek said) - it's more that they are demanding
. Each polyp needs to receive food, in order for the coral to thrive. That means using some sort of feeding hat (easily made from a plastic soda bottle) - or somthing similar. It also means finding the time to feed them 3x-4x/week, and not tiring of it.
They don't care about light (don't have symbiotic algae), but seem to fare better in dimmer conditions where algae isn't as prone to grow on them. They do however, like at least a moderate current.
They will eat adult brine, mysis, finely chopped krill, etc. If fed consistently, and often enough, the coral will begin to produce new polyps. However, if they are not regularly fed, the polyps will no longer open and the coral will eventually die.