Never used Rumble, looks like a make shift copy of YouTube.
I just picked a random video, clicked on the down arrow with M3U8 and video downloaded.
I had to click the download now, and then it showed the same options to download now, I noticed it did start downloading so just clicked cancel.
It is not working as it should, but I did get a video downloaded.
If still issues, describe what video hopefully 2, what issues you get and possibly attach a steamfab.log. (sometimes screenshots help too)
Being that it looks so much like YouTube, I would think SF works for most videos.
Or also wait and see if someone else who uses or has used Rumble to see if they know more.
