Car undersealing stems from the 1970's approach to rust protection in the UK and involved the application of a very thick, heavy and hard protective coating which didn't contain any rust inhibitors. This solution was also prone to problems in that the treatment often dried out and because of the thickness as these coatings dried out they pulled away from the surface of the car and created a water trap allowing moisture to get in and create a rust trap.
Our modern methods of rustrpoofing involve applying a much thinnner and more flexible layer of rust protection which also contains rust inhibitors which stop the start and spread of rust.