Topic or subject which is not easy to discuss/understand. It may also be used in case of an issue from which it is very difficult to draw valid conclusions from, due to its complexity or the lack of expertise on the subject from the speaker, for example.
On the subject of comparing Led Zeppelin and Deep Purple in terms of guitar virtuosity, I think that's a rather nettlesome issue.
by Xabier Rubal January 05, 2006