Trolltech used .use file in the 4.0, because cmake don't supported export/import in that time. In last versions Qt use .use-file for backward compatibility. I don't know the reason why Qt5 switched to export/import. But in any case we shouldn't use uniform style. Lets discuss what is better export/import or use-file.

Advantages of export
 1. This is out of box.
 2. Fix me, what else?

Advantages of use-file.
 1. We can use "if" operators in use-file, and have one file for both versions of Qt. As result programmer what use our library may write less code.
 2. We can place some cmake macros/functions in the use-file. Now the qtxdg hasn't such code, but lxqt has.
 3. We can declare directory for the public headers, is this posible through cmake export?
 4. Fix me, what else?