Any coatings that harden can have openings where not thoroughly applied or where damaged by road debris. Once those openings are there water can get in, get trapped and accelerate corrosion. Back when vinyl tops were popular I saw lots of rusted out roofs on cars. The same principals apply to the underside of cars as well.
Back in 1978 my father ordered a new Chrysler and paid extra to get it undercoated. After driving it home he looked under it and observed that it wasn't undercoated so he took it back to the dealer to get what he paid for. The service manager's response was "I thought that was standard equipment." To which my father responded "I have two complaints. First, I paid extra for a standard feature. Second, I still don't have it."
The responses that you get from dealers can be hilarious if you have the patience to put up with them.