Here in Wisconsin the frames usually rust apart and become unsafe to drive well before the drivetrain fails. My last car made it just shy of 200k before the strut mount rusted through the unibody.

Now that I coat everything underneath with Woolwax I'm seeing much less rust, but still something you have to stay on top of.

Corrosion is still a problem anywhere they salt the roads (i.e places it snows). Much less of a problem than in the past, but still an expected thing at a certain vehicle age.

Normal driving results in tiny rock chips over enough miles and all it takes is the tiniest chip for the corrosion to take hold. Super common above the rear wheel wells, at least in my area. Under body corrosion is pretty inevitable as well because it isn't painted, and even if it were it still takes a lot of abuse from road debris. Coating can help significantly (Fluid Film, Woolwax, etc) but you can't coat everything 100% so it still happens.