If boiled icing is over mixed, too much air will be incorporated. The icing will be thick, full of air holes and impossible to spread. Use boiled icing as soon as possible after whipping. Boiled icing sets up quickly and is almost impossible to spread after cooling. It's worth trying.
Dry fondant is similar to powdered sugar.
It is easy to use, because all you add is hot water. Scaling is quick and simple with dry fondant. Wet fondant is extremely thick. It sometimes takes an effort just to dig it out of its container. Scaling is difficult and messy because the fondant is sticky and only small amounts can be dug out to be weighed.