I did undercoating and rust proofing for GM about 15 years back. Things may have changed since then, but generally, the tools and system were made for speed and bare minimum coverage (meaning, do it just right... no more than that.. and if you miss a spot.. nobody would really know). Bear in mind, this wsa a dealership, I was trained by the undercoating/rust proofing supplier and it had nothing to do with GM itself. The undercoating was sprayed on using the same technique as a paint gun, and it was not meant to do a complete coverage, just "prone" or noticable areas. Likewise, the rust proofing was a wax/oil compound that was sprayed inside of all panels with holes.
What a lot of people who get it professionally done dont realise is that you are usually supposed to follow a maintenance schedule (having the car checked every 6 months) so that problem areas can be solved before they are problem areas.. lol. For my 88, I am rustproofing the car myself, but with the interior totally off and paying attention to where the metal forms a joint. Where the wheel well meets the quarter panel on our car is a bad area for rust, but a lot of people never get to see the holes in the front of the wheel wells (rear) where the undercoating was perforated. This allows for water to get into the panel behind the door and crawl into the rocker plates. That rust is not seen until it fails.. especially when you have plastic covering the metal in front of the wheel.
Hopefully professionals now have looked at each car as an individual, rather than an overall job. They all have different quirks (as seen with MKII and MKIII Hatches. MKII will rust.. the MKIII isn't so prone to it.
Most people are suprised when they strip the interior out and find rust on the strut pillars or the metal under the hatch window.. never even thinking that perhaps the seal leaks there (you wont see it unless you strip your interior out .. )
I believe the statement has been that 85% of cars that are not on the road, were removed by rust.