My experience has been less than stellar. Granted I don’t do a lot of big cakes either.
Last summer I did a royal icing over a buttercream (Wilton recipe) and it looked amazing. Left it sitting overnight and when I moved the cake in the morning to take pictures and to box it up for delivery, every single one of my piped lines fell off the cake. Every. Single. One. This cake was being transported over 3 hours away so I knew that redoing the piping wouldn’t be an option.
This is what it looked like the first time I moved it:
Every time I moved it more fell off.
I’m not sure adding meringue powder to the buttercream would have prevented this or not.