Sometimes it is and sometimes it isn't. I've seen people put them in brand new tanks and live but you are running that risk. Nothing happens fast in SW
and patience needs to become your best friend for the best results. There are many different microorganisms that need to develop in your tank over the first year before it is a fully established healthy reef tank. Also, when a nem dies it can nuke a tank (kill everything in it) since it will release all its toxins into the water so youll be risking an expensive nem and anything else in the tank.