Currently audioac3 command/line only applies to source AC3 audio. This means that any other codec automatically skips this line and uses the -audio command/line instead.
This has the side effect of creating unnecessary heavy files when the input audio is something like Flac 2.0 or EAC3 2.0 etc. I went into more detail here - Best way to handle a folder with multiple audio formats
It would be great if the audioac3 would work as a catch all for all audio formats. This would allow one uncomplicated (no fall through etc) profile to handle all input audio formats the same. While i’m sure that there is a good reason why this is limited to AC3 only, it would be great if it wasn’t. Simply put, at least in my case, it’s irrelevant what the input codec is - handbrake or ffmpeg can handle all of them with ease. It’s just a question of trying to avoid files with 2 channel output at unnecessarily high 5.1 bitrates etc.