Google operates a transcoder API which I suspect is just ffmpeg under the hood, and if you assume that they accept any input file, they really can't afford for decoders to have security vulnerabilities. Of course, then Google should be coming with more resources and not just filing bugs because it's Google that has the unusual use case.

I could see a compromise where if there are obscure codecs that may not be as secure, FFmpeg would present a warning before loading the file. This way, the user would have the option to decide whether to load the file or not. By default, potentially malicious files would not be loaded, which could prevent them from being used as part of an exploit. This seems like a reasonable compromise.

>rarely anyone ever uses

It's enabled by default so all that's required to exploit it would be to construct a payload file and name it movie.mp4