It sometimes works for me on most websites (except youtube) .
Very unreliable, specially for slow connections. Suddenly crashes or claims download is finished (but it's not).
You need to check every video manually if it has fully, not corrupted downloaded. Re-starting downloads of such vids is a PITA, sometimes it just don't.
Regarding the "companion app": I can see in the ressource monitor that this app does not send data, it only receives. Means: I didn't catch it spying.