I am trying to see all the comments a user has made.
For example, I Googled:
But apparently this does not work any more. Is there an alternative?
After the integration with Google+, YouTube's comment section is indexed differently. There no longer is an "/all_comments" section for each video. In order to search for comments on Google, you'll change your search query to:
"Username"* ago site:youtube.com/watch
I made a video explaining how to do it here: Search YouTube Comments With Google