I wrote about some stuff here https://paul.kinlan.me/ffmpeg-ideas/ when building https://paulkinlan.github.io/deviceframe.es/ (I do a lot of screencasts and need a simple way to scale this out to other people that doesn't involve installing ffmpeg and scripting)
EDIT and Note: Also I just saw your comment about videoconvert.js
Have you compared the speed of Android javascript ffmpeg vs PC native? What happens with the battery?
I think mobile is a secondary consideration in this at this point. Considering it is connected to the internet archival efforts and simply ensuring the survival of numerous video/audio formats I imagine they are fine if it isn't efficient but simply effective at this point.
Edit: on second thought, you might not be commenting on the VLC.js thing, I haven't read about the ffmpeg js stuff ;)
I've asked about