I just restored my backups a few weeks ago when I bought a new phone. It doesn't work for all apps (I knew it, it's far from the first time I do that) but I am satisfied with the results.
It does help sometimes for retrieving some data manually by digging through the backup files (used it recently for retrieving all invoices from a service that were otherwise only available one by one through a painfully complicated process).
Not sure if it restores well because I don't use WhatsApp anymore, but I checked in an old backup and somewhere in the files there's an SQLite3 database "msgstore.db" with a "messages" tables that contains messages.
