I would stress the "meaningful" part. I assume that every country in EU gives out CC0 (or so) licensed data for at least geocoding that have the same EU-wide high-level hierarchical format. Problem is that this hierarchy does not map well to OSM tags (which is one instance of problems caused by using tags instead of layers) and often also does not map well to local users' expectations (ie. converting the object names to/from handwritten address is non-trivial problem). Also sometimes the government provided data tends to be stale.
Stale or not, this seems like a ridiculously complicated problem without some set of rules/boundaries to govern it. I.e. You're doing a diff with spatial data that not only differs in the spatial component, but in the semantic/descriptive domain as well.
> I assume that every country in EU gives out CC0 (or so) licensed data for at least geocoding that have the same EU-wide high-level hierarchical format.