This is quite exciting because I wouldn't have to install anything to use this at various facilities I might be working at. Although I tried to test it out with the most common things I might use FFMPEG for.
First up I tried to "replace" audio with something else, there was no replace function. Then I thought, "cut"-ting without encoding (although usually I want to cut a section out and replace it which won't work here) is something I also do to save time since most professional apps won't do it. Sadly there was no recognition of the word "cut". I also thought I'd try "remove"-ing something, which admittedly is basically the same as either cutting or replacing since I'd basically be removing audio to replace it with something else or removing a section of video without needing to reencode which is the same idea as cutting, just a different keyword, but again that didn't work. Another thing I often use FFMPEG for is encoding "pro res" on windows machines and there appears to be no recognition of "ProRes" either. Another thing I sometimes do that I thought could be a good test is "remux"-ing. This is a bit of an edge case admittedly, but I just wanted to find something in my usual list of needs that this might do, there appears to be no mux, remux, or multiplex keywords recognised either. Also commonly when I'm remuxing, I'm also mixing down 5.1 to stereo and sadly I couldn't find any "mixdown" or "mix" keywords recognised either. I did find the "normalise" command though, so that's good.
I don't mean to shit all over this because I think it's a great idea. When I use FFMPEG it's a frustrating and finicky command line experience and front ends inevitably have to limit what it can do in the interests of simplifying its use. I often end up carrying around strings of commands in text files because it's too hard to remember the exact syntax for the things I often need to do and I love that this has the option of generating those commands for you based on plain English keyword requests which is handy if you have FFMPEG installed locally. Although that's another thing that I wonder about with this, if you're having to install FFMPEG to use the command line output from your app, then at that point you've already had to cross a threshold of inconvenience where this is only marginally helpful at this point and while I see you can upload the input media, FFMPEG is primarily for dealing with video and the time it would take to upload anything other than a short and already compressed file would make this rather prohibitively impractical.
Most likely my usage of FFMPEG is eclectic so I hope this helps out people with less unusual needs, but for me at least, I only really use FFMPEG for unusual situations because I specifically want to do something that professional apps don't do or because the need is simple enough that I don't want to open an editing app. The main thing I'm doing with FFMPEG is making minor changes to a video that has a mistake of some kind but I don't want to waste time reencoding. That's a common thread amongst all my failed tests. They're all about a hypothetical scenario where I have a huge file that I've discovered has a small mistake in it that I want to patch quickly without a reencode, or needs a second version with a different audio configuration but the video is fine, or where I want to reversion it to work on stereo equipment but again leave the video unchanged. I can do all of that one way or another but where FFMPEG comes to the rescue is allowing me to do it without touching an editing app and avoiding reencoding wherever possible.
If I just had something very bog standard to do like convert something to h264 I might use FFMPEG to do it for convenience sake but then if I'm having to upload it first then wait for transcoding and then wait to download the output it'll become more hassle then just using a pro app I might have access to, not downloading FFMPEG in conjunction with a GUI.