Actually, I pipe royal icing on Swiss Meringue Buttercream all the time, and have NEVER had any problems!! SMBC sets up so much harder when refrigerated than traditional American buttercream, which probably helps… that, and I always put my cakes on stiff stiff boards, so there’s no chance of the buttercream shifting.
I’ll attach a few photos here ~
The top tier was piped with royal,and I used the brushed embroidery technique on the flowers.
The bottom tier’s swag was royal icing.
The elephants were royal icing.