sometimes they can be ok. sometimes they can be unsafe. For example, a box section with a box inside it like often found around the rear quarters in modern hatchbacks, when they weld a new rear corner on they can only weld the bits you see, meaning the inner box section remains unwelded after being cut out... but you'd never know cause from the outside it looks welded.

Best way to deal with this is inspect it yourself before damage and make sure it can be rapaired to how it should be, or make sure there are pics of the repair showing things like this are worked around.