I introduced a purple s. gigantea and a green s. gigantea into my three month old 150G reef with zero issues. It really is about the health of the animal. Like someone else has said, they ship very very poorly, and usually end up getting a bacterial infection which is the cause of their demise.
When trying to keep something like a s. gigantea, make sure you have a hospital tank ready, and follow Minh's (RC Gig Guru) cipro protocol. This medical protocol is easy and has saved even in the last year, several dozens if not over 100 s. gig's lives. Mine have now been in the tank about 8 months and are thriving. They are beautiful animals, and haven't lost a fish yet to them.
They aren't all together hard to keep, it just takes dedication and a love for a very complex, and delicate animal. S. Haddoni is easier and hardier, but very much still have cipro on hand and follow the same regiment. Those, are more of a notorious fish eater.
Good luck.