This Macworld article has a good comprehensive overview of the different installation options when upgrading the operating system. What I found particularly interesting is the following comment about the archive and install option:
For example, if you used the Archive and Install option to preserve user accounts, the files inside the Shared user folder (/Previous Systems/Previous System 1/Users/Shared) don’t get moved.
For my setup as well as the one I just set up for my parents, this is of critical importance, because I keep all my digital pictures and music on the shared drive for easy access from all user accounts.
It is not clear to me, why the shared user folder does not get copied automatically. It is not consistent, and certainly not expected.