Youtube - I get a 'File not found' error message when I try to download any youtube video.
DailyMotion - The download completes very quickly (in 1-2 seconds) irrespective of video size/length and the video does not show up in the library. Also the 'Download' button next to the video disappears.
I tried the latest version - 5.04 - on Windows Vista. I also tried the master nightly build for Windows (2013-02-19-master-nightly) from this link - http://nightlies.pculture.org/. The bug is present in the nightly build as well.
The problem is not with Miro. The Youtube videos that I was trying to download were marked as "restricted from playback on other sites" by the owner and thus Miro wasn't able to download them.