The software product-line approach (for software product families) is one of the success stories of software reuse. When applied, it can result in cost savings and increases in productivity. In addition, in safety-critical systems the approach has the potential for reuse of analysis and testing results, which can lead to a safer system. Nevertheless, there are times when it seems like a product family approach should work when, in fact, there are difficulties in properly defining the boundaries of the product family. In this paper, we draw on our experiences in applying the software product-line approach to a family of mobile robots, a family of flight guidance systems, and a family of cardiac pacemakers, as well as case studies done by others to (1) illustrate how domain structure can currently limit applicability of product-line approaches to certain domains and (2) demonstrate our progress towards a solution using a set-theoretic approach to reason about domains of what we call n-dimensional and hierarchical product families.