That's an interesting question. They only depend on a single library, but I wonder how much code is really their own. I found it curious, for example, that there is a dedicated mp4 joiner (I mean, if you already have ffmpeg, there is probably no way you can do it better yourself).
That is interesting, huh, yeah they list ffmpeg as a dependency so I wonder what that didn’t cover for them.
Though there are some cases where using pure ffmpeg is just to difficult or impossible. Recently i had such a case where I wanted to merge multiple video files from my GoPro (they come out in 4GB parts if the video is longer than that), but while keeping / correctly merging all metadata such as date / accelerometer / gps / any custom binary streams. Ended up using this and worked great https://github.com/gyroflow/mp4-merge
https://github.com/soimort/you-get/blob/develop/src/you_get/...