If you don't perform a task frequently, a discoverable interface (e.g. GUI) is dramatically more productive than reading documentation.
In ffmpeg's case, I would be looking for a GUI beyond just "here's all the command line parameters, only in a form"... the hardest thing is taking something I already know what I want to do, and encoding it into the command line, because the way to encode a graph into a command line is not obvious. It is easy in principle but there are so many degrees of freedom in how it is done that I can never remember it all. This is a perfect time for a visual language.
IMO the best option is the best of both worlds. A GUI that outputs the CLI command along with good documentation would be ideal.