What about collaborations? The nested hierarchy implies a directed tree structure, but the relationships are really a graph. And I've personally found no more insidious of a problem in data design than trying to shoehorn a graph into a directed tree.
The relationships in URIs are abstractions anyway. The data is not actually stored at /artist/8/albums or /albums?artist=8. Both are abstractions of whatever underlying system is used to store the relationships.
