I can confirm this issue. I had files like:
file.rar
file.r01
file.r02
file.r03
...
The files were compressed with RARv3 so unrar-free (or the GUI) could not
extract it at all.
With theunarchiver I could extract only "file.rar" and get a small part of
the file. None of the other files would give me anything.
I tried cat'ing, extracting them all with *, extracting them separately but
with no luck.
IIRC, I managed to extract the files on a Debian box with unrar (non-free).
Here is a possibly related (but somewhat old) issue on TheUnarchiver's issue
list: http://code.google.com/p/theunarchiver/issues/detail?id=238
