Meringue powder is a powder developed to easily make meringues. As such, it contains egg whites and sugars in a predetermined ratio. It's perfectly fine to use for making royal icing. But, it does have a major disadvantage. Since it contains both egg white and sugar, your creative freedom is a little limited. You can't adjust the egg white content without also impacting the sugar content. That said, it also is more flexible than using liquid egg whites.