In a one-word answer, it''s RUST. You will, at some point in the future, breach the protective barrier coat on the keel and voila, the rust begins, regardless of whether the boat is kept in salt or sweet water. Once the rust begins it will always return. In addition, the keel must be larger if made from CI than it would be if it were antimony lead, which means more wetted surface. Cast iron keels may not be as resillient or forgiving as a softer material such as lead in a impact situation. The reason they put cast iron keels on boats is because it is cheaper not because it is better. I would not own a boat with a iron keel, just a personal thing. Does this model you refer to have an iron keel?