difficult on either a lathe or a mill ...
a keyway slot in a pulley is the
first one that comes to mind --
At that point you pull out a shaper :-)

A broach would be my first choice!


Other than keyway slots and dovetails, the biggest advantage to the
hobbyist of a shaper is the ease of creating special bits ..

You know, I ordered a corner rounding endmill for a job. I could have
ground a bit for the shaper and saved a few bucks.. sigh. Oh well. next time.