Impaction typically occurs when the dog ingests too many green bones. The majority of the bone is not digestable, and the softened fragments are passed in the stool. The stomach's acid softens the bones to a "rubbery" state and extracts some nutrients from them, but they are never completely broken down. The danger is when many softened peices clump together and form a plug (impaction) that cannot pass through the small intestine.
I think it is one of those things where one must be prudent and observe the size and quantity of bone fragments ingested by the dog. Of course some dogs may not produce enough stomach acid to perform the task, or may swallow the bones in very large pieces.