The table above is only a rough sketch, please improve.

Corresponding Elisa task: T8195
I tested a little bit, and using toString in QVariant for a StringList with one entry returns the string, with two or more it results in an empty string. Judging from the code, Baloo succesfully reads StringLists with one entry, but silently fails with more entries. This is the same Elisa is doing before D11340

In T8196#132926, @astippich wrote:

Corresponding Elisa task: T8195
I tested a little bit, and using toString in QVariant for a StringList with one entry returns the string, with two or more it results in an empty string.

If you've got a string list use .join(", ") or so, for QVariant::StringList use .toStringList().join().

Judging from the code, Baloo succesfully reads StringLists with one entry, but silently fails with more entries.

Yes. And as far as I understood it parses those token-like items (e.g. subject, keywords) like plain text. Imo it should use them as a whole. E.g. a keyword 'otto' should not be found by the search term 'ott'. But that's only my view.

Nearly forgot: Please complete the list above.

With this multi-value-map pattern in https://phabricator.kde.org/D10694?id=30858 baloo-widgets is having trouble and displays only the first subject.
I'm trying to adapt baloo-widgets, but it is quite difficult and takes a lot of (ugly) code or a major re-write.
In contrast: If kfilemetadata would return string lists the adaption could be done with 2 lines of code. I'll have to look, but my guess is adapting baloo to handle string lists is easier.
Preliminary conclusion: I very much prefer the string list version.