The most inherent problem child in the G-body lineup is the vinyl top roof on these things. No matter how meticulous you are with it, unless you keep it in a de-humidified room all its life, it will likely rust to some degree under the top. From what I've seen, they used some sort of padding under the top, which collects and distributes water across the non-painted surfaces of the metal underneath. And it just sits in a water bath. Caught in a rain storm, or just normal washing the car will do it. If you're lucky, you won't get too much other than surface rust. If you're not, you'll have pieces of top come off in your hand when removing the vinyl.
They look great and formal and all when new. Nice appearance feature in general as it adds some classy aura to the car. But it's a cancer death-wish for the car. They never ever considered anyone keeping their cars for more than 10 years when they built them, so longevity wasn't built-in to their plans. Unfortunately, vinyl tops are almost a guarantee of some degree of rust issue underneath. Just expect the worse, and be glad if it's just surface pits when the time comes.
