I would say it’s all about stability just like any sps tank but in all honesty I’ve never been able to keep one of the designer nems alive for more then a few months.
About 10y back I did a 180g all nem tank. I had all types of nems. A giant ritteri, 2 carpet nems, 30 something RBTAs, 3 true Fuji red rose nems(what people like to call black widows today), a few long tentacles, a half full of rock nems, and even a few sebae’s. All them thrived except for and of the dam CSB BTA. So I gave up on them years ago. I refuse to spend another penny on a CSB.