For what its worth, my sw20 got dealership installed undercoating when new. Drove it year round, it rains for like 4 months straight here and I used to go to school on a mountain which had snow every week in the winter. It was also parked outside for the first 8 years of my ownership. Still rust free and mint. It
is ugly though, especially after 33 years. It was originally cream coloured and it's now a good mix of dirty and stained.
On the flip side of things, the subframe here is just the factory underbody paint, no undercoat in that area. It too is okay so who knows if the car would have been also okay without undercoat.