You know — I really couldn't tell you if that plugin was supposed to actually do something or it's just something you look at (like any other meter) to figure out what you need to do with the audio by other means.
I thought LUFS was supposed to be the goal in broadcast standards...FCPX is used in broadcasting. What other reason is there to make an AU other than compatibility with Macintosh? Perhaps Jeroen doesn't really know what he's doing. Where there's an audio signal, and the plugin is designed to work as an AU (the audio should be nearly, if not the same as it is in Logic Pro) then the plugin should be able to do its job; no excuses.
Personally, I can't figure out the purpose... well... I can, but I think the fundamental basis of it is "fake science." Its purpose seems to be a sound squeezer -- funneling everything through the filter to get all audio sources to sound "at the same levels"... so... it's a form of compression - a kind of meat grinder for audio. To use a fi-dolla word, that's anathema to me. It doesn't take anything else into account other than the peaks, and the lows and spits out an average which is then "normalized" (another bad word)... still not seeing (or hearing) how it relates to "Loudness" (which is a perceptual concept that is different for every human on the planet and basically the very reason for volume controls on sound delivery devices.
If it were something "real" I genuinely believe Apple would have included it in its professional audio apps (Soundtrack Pro, Logic Pro.) [Apple is probably the very reason for the death of DVD simply because Blu-ray Disc is a "bag of hurt" to paraphrase Steve Jobs—and he promptly pulled the plug on all optical drive support.]
But... maybe that's just me. I think the tools available in Logic and... yes... FCPX are some of the very finest audio tools available anywhere. Perhaps the four greatest (targeting your specific current need) are: Adaptive Limiter, Multipressor, Linear EQ, and the Multimeter (this is in Audio > Specialized > Logic section in FCPX.) It handles RMS, has a goniometer, and correlation meters (learn how to use it and you can make all your audio sound *amazing*).
I apologize for the rant... it's just been very frustrating trying to uncover the real purpose for the use of LUFS... and the lack of any worthwhile information, either for general knowledge or how to use the so-called tools created for it.
If you're being asked to use it (you have my heartfelt sympathy), find out if there is a specific reason or just something the asker "heard about" and thinks it might be a good idea. Otherwise, whichever broadcaster that is using probably pushes everything through a LUFS plugin upon broadcast anyway and probably not anything you should have to worry about. It's not in standard use in the US (yet.) You shouldn't be expected to use something nobody seems to truly understand.
