Hard cast lead is not going to be hard enough to "wear down" your barrel. The bigger issue would be accuracy based on the way the lands and grooves are cut in the rifling. Heat is what wears a barrel down so, smokeless loads with slower burning powders keep more heat in the barrel and over time will cause erosion from the throat of the chamber forward. With black powder the problem has to do with cleaning mostly whether it is done correctly, too often or not enough. The advice for a softer bullet is sound since that will give you the best performance given the original type of ammunition it was set up to shoot.