The enameled steel's finish cracks/chips easily if something is dropped in it, like a shampoo bottle. Then, it rusts. The primary reason for that is that the tub is fairly thin. Cast iron is much more durable since it is much thicker, the finish is supported (now, you can chip it, too, but it is alot harder). The reason the fiberglass tubs start to look nasty fairly quickly is that most people don't set them in cement topping mix, mortar, or plaster. If that is done, then they don't flex, and can remain looking good for longer (not as long as a good cast iron, though by any means).