I had some trouble getting quicksync to work on urraid with FileFlows. The problem was that I have a 12th gen processor and it apparently requires libmfx-gen1.2 when using ffmpeg with qsv.
If you get this error and have a 12th gen processor, you can temporarily fix it by installing the package inside the docker image.
Error : "Error initializing an internal MFX session: unsupported (-3)"
Fix: "apt install libmfx-gen1.2"
However, any updates will revert the change and you'll have to re-install, unless this package can be safely added to the base docker image.