License may be GPL2+ if certain components are included, see https://ffmpeg.org/legal.html
(Also mentions potential patent restrictions that may be imposed by third parties - see references in :fsf for discussion of the general issue)
MP4 in Quicktime
MP4 (H.264) files that can play in Quicktime
...but the main reason to play in Quicktime is frame-by-frame scrubbing.
Other options include :autodesk:rv and FIXME: free software replacement with this feature
https://trac.ffmpeg.org/wiki/Encode/VFX (specific issues for :animation / VFX)
Note complaint about :openexr support. ffmpeg supports decoding only, not encoding (see https://ffmpeg.org/general.html#Image-Formats). So :imagemagick might be more appropriate for that specific conversion.