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.
Doesn't apply to well-designed GUI programs. In good GUIs features are discoverable, you don't need prior knowledge or documentation to use software.
Experience speeds things up, e.g. keyboard shortcuts are faster than menus or buttons. That's optional though, one can still use the software without these shortcuts.
For example English orthography is a horrible mess. The inventory of squiggles needed to write English isn't too bad, but the correspondence between the words you know and the correct sequence of squiggles to write them is unnecessarily complicated. No benefit accrues to us from this, and in some other written languages it's much easier.
I'd place ffmpeg somewhere in the not-bad but not-great part of the continuum. As with English orthography of course the problem is if you change things with the intent to make them better you actually introduce a cost for existing users which may be impossible to sustain.
Kinda like running into an old friend you haven’t seen in decades.