Agreed. My Bonneville is in mint shape despite being driven in the winter for over 2 decades, the only stuff that rusted was the floors due to salt from the inside, and a part of the frame near the steering box which the undercoat had missed.Undercoat is better than no undercoat, but, it's not a silver bullet for rust.
Realistically you need to spray it before the car gets driven around to keep dirt out from underneath. Spots get missed. Coating cracks over time and allows rust to fester and rot behind the rubberized coating. Not everything can be sprayed if you want to work on it decently. You need to drill holes and add plugs in the body which hold up OK in door jambs but can come out underneath. So on so forth.
Then consider the mess. Gets even worse with coatings like fluid film that wear off.
So it’s definitely not foolproof for sure, but to me people exaggerate southern cars being better than northern ones. Really depends on how the car was maintained and driven imo.