Two examples I remember researching Ruby for and ended up giving up: hardcore statistics stuff (I'm using using Python's scipy.stats now) and GUI toolkits (yes, there are some toolkits available, but nothing like Python's official bindings for gtk/qt).

It's hard to remember more examples. I remember having problems even with FastCGI on Ruby, but I ended up not writing the code at all so that won't count. Also something related to parsing emails.