I had a reasonbly big emporer (about 7-8") with a RBTA.
This angel was a well experienced coral muncher - just about every soft coral you can think of; devils hand, sacrophyton, GSP's, regular star polyps, etc. - but never touched the new.
The nem had a pretty assertive pair of clarkii clowns in it, but to be honest if the emporer wanted to eat it, these little guys wouldn't have stopped him.
I don't know much about nem's to be honest - I don't know if RBTA's have a particualrly strong sting? If it does, perhaps that detered the emporer. This particualr nem was capable of catching and consuming a whole lance fish (silver side) or prawn (2") within a couple of mins - so perhaps it was a "strong" one.....