I never use anything other than real ribbon on my cakes, but then most of them have been fondant covered and in that case you secure them with a little dab of royal icing at the back, then overlap the ribbon and use another dab of royal icing. If the buttercream is crusting I don’t see why you couldn’t use any type of real ribbon when it has crusted. However, a thicker type of ribbon like a grosgrain should be thick enough to prevent any moisture seeping through and always looks stunning.
