Powders rarely contain any useful vitamins or minerals, either. When made with milk they often acquire a bit of calcium; water-based mixes typically have little to no vitamins unless they are specifically labeled “enriched.” This is in contrast to egg-based custards, which often have a lot of protein, Vitamin B12, and iron, among other things.
People who suffer from certain food allergies often turn to powdered custard when they find they cannot eat the original version. Most powders are free of gluten and wheat-based ingredients, and the majority are also completely egg-free. People with serious allergies would be wise to check the labels of specific products before consuming them, but powdered custards are usually safer.