This add-on is open source (you can check the github), requires ffmpeg and native client (both also open source), and if you take the time to properly install and configure everything, take into account that the videos for download still show as "video only" and if you click on (1 of) them, AND WAIT, it will download video AND audio files and merge them appropriately. Use ctrl+j to watch the progress when you click a video and take into account that on your disk, the files all show 0 size until fully downloaded.

The most transparent add-on I found, and it works flawlessly, so I'm giving it 5 stars. Thank you developer.