Why link-shared events are invisible without this
There is a category of event that no event dataset contains, and it is a large one.
A school concert, a sports club fundraiser, a church fete, a small amateur production - these are ticketed, they have a page, and they are shared by link rather than listed anywhere searchable. Someone building a local calendar from the major platforms will not find them and will not know they are missing.
A booking platform used by those organisers holds them, and its published sitemaps make them enumerable. For local media, councils, tourism bodies and anyone claiming to know what is happening in a town, that is the difference between a calendar and a partial one.
The second point is the country split. Sitemaps are separated by market, which makes coverage checkable per country rather than inferred from addresses - useful because the platform's penetration differs substantially between its three markets.
The third is the metadata unevenness, which is inherent rather than fixable. Small organisers write short listings. Recording completeness as a field is the honest handling; filtering on it would remove the events that only exist here.
The fourth is that this is a booking tool rather than a marketplace, so there is no ranking, no promotion and no editorial selection - the listings are simply what organisers created, which for research purposes is a cleaner sample than a curated platform.