complete data export
Currently it's possible to export only urls of subscribed feeds. It would be desirable to be able to export the data that the service stores for the feeds and also user submitted metadata (folders, stars). Like with Google Takeout... to not be locked into the service.
I'm planning to implement it but can't say ETA.
Folders are already exported in OPML file.
As a workaround for exporting of starred/tagged items you could create public feed (right click on "Starred items" => Public feed => Enable) and save it (you might add ?n=1000 to URL to save more items). It will save items as RSS feed.