Having trained rustproofing applicators in a distant past life, I will not being have it done to my own car.
Part of the process involves drilling many half inch holes into various areas of the car, leaving bright, shiny metal to start rusting (under a plastic cap). Also, the skill of the the people applying the goop into the nooks and crannies of where ever they are spraying varies widely. Plus, cars today are made with much better steel, often zinc coated and already rust resistant.
I live in snow country, have a 1997 that is just now starting to show some small rust spots, but almost all are from stones and grit hitting the front. The 1990 I sold for the Volt had a bit more, but nothing rust "proofing" would have prevented. Nether were rust proofed.
Instead, I had the clear bra treatment applied to the Volt. In my opinion, a much better value and less a waste of money. Just my opinion. YMMV.