I'm sure it's normally case by case, Here where I live we don't suffer rust issues as such caused by salty roads, But more so the rear subframe are common to rust here as they are often used as boat tow cars.
To remove the rear subframe for instance is a major job, To replace the bushes in the subframe again is another undertaking and you need a special workshop press to do this, I Beleive the rear subframe alone is about 1-2 days labour.
Perhaps if the rust is just surface you may be able to use 'killrust' spray and a bit of zinc paint?