First if you use Youtube there's this passage you agreed to.
"You shall not download any Content unless you see a “download” or similar link displayed by YouTube on the Service for that Content."
https://www.youtube.com/static?template=terms

To press their point Youtube will change and break downloader apps.

So there it is, download apps are doomed to fail.

Since this invariably results in spam replies, links are now moderated in the forums.