Youtube-mp3.org is the 900 pound gorilla of Youtube to MP3 converters, at least as far as traffic is concerned.
The downside of this popularity is that Youtube-mp3.org has attracted the wrath of Google, which have banned the site from accessing Youtube’s servers.
In other words, the first search result in a Google search for “Youtube to MP3” is broken — and Google broke it. Go figure.
I think, as an operator of such a site, you should be prepared to having to work around all kinds of technical challenges.