Shellac has its uses in guitar finishing. For instance, it's a fine barrier coat if you want to switch horses in mid-stream as it'll stick to damn near anything and damn near anything will stick to it. So, if you've got a poly finish you wanna shoot some lacquer on, or vice-versa, a coat of shellac will allow you to do that.
But, as a primary finish, I don't know that I'd wanna use it unless I didn't have any choice. It's kinda soft even when cured, it scratches/marks easily, it's sensitive to water and heat and will dissolve in the presence of alcohol, and so on. For an item that's going to take a lot of handling and exposure to various fluids and environments (like a guitar), it's not a great choice for a primary finish.