I know royal icing dries hard for flowers. Seems hard enough to crack teeth when eaten alone. But....can I pipe small decorations on the side of a buttercream cake and it be a little soft or will it still be crunchy?
The grease in the BC might break down the RI if piped on directly. Someone with more knowledge might be able to tell you more, but I HTH!
I pipe with royal on buttercream all the time. It holds up fine. It does dry crunchy so it breaks when the cake is cut.
RI is great for vines or stringwork on BC -- I find it a lot easier than BC stringwork. And, as crisc23 mentioned, it does "shatter" when cut, so you don't notice it much when eating it.