I heard stories of incriminating stuff for higher-ups disappearing from archive.org.
My main use for archive.is is for sites that somehow cannot be archived (a message will show up mentioning this site cannot be archive or something along these lines).
archive.is is generally pretty good in forcibly attempting to get an archive, if the HTML doesn't work, the screenshot will work fine. Although archive.is doesn't seem to handle gifs/videos.
They respected exclusion requests after they stopped to respect robots.txt. I don't know their policy for new exclusion requests.