If the hg root contains multiple projects, usually there are some files which are outside of Eclipse workspace (most prominent example: .hgignore).
It is impossible to change this files in Eclipse or to commit changes or to see diffs for them in outgoing/incoming view.
Very bad: in case such files are modified, and you have to perform some operation which requires clean repo state, you can't even see which files are changed and can't revert them. Etc.
As much as possible of the standard MercurialEclipse features should be applicable to workspace external files.
I have a partly working solution with most important things implemented (commit/revert/diff/sync view).