Moorish Idols can be tricky to keep, but there are quite a lot of people who have recently been having good success with this species. Personally, I think fish food has been getting a lot better over the last several years, and their reputation of being impossible to keep is based on old stereotypes. However, they're still a fish that's probably best left to advanced aquarists.
The species is definitely not being overharvested - they are abundant in the wild, and because they have a reputation for being difficult to keep they're not collected in very large numbers.
Yellow Tangs are not being overcollected, either. As a matter of fact, they are one of the best studied aquarium fish in the world, and it's been proven, with solid scientific data, that the fishery is sustainable and has been for the past decade. The complaints about the yellow tang fishery come from people who have moral problems with keeping fish in captivity - whether or not the fish are actually endangered isn't the issue here.
Some species of linckia stars may do better in captivity than others, but I've personally kept spotted linckias in my tank for several years with no problems.
