As a FFMPEG API user (e.g. through libavcodec etc.) I would definitely not say "simple". It's constantly breaking stuff and deprecating features from one version to another, and basically requires reading the source constantly to make sure of what's happening and on which backend / API each function can operate. Just today, when I was trying to implement vulkan video decode in ossia score (https://ossia.io) :
Copy data to or from a hw surface. At least one of dst/src must have an AVHWFramesContext attached.
int av_hwframe_transfer_data(AVFrame *dst, const AVFrame *src, int flags);
Well unlike what the very first sentence of the comment block hints towards, it actually is only implemented for host<->device copy, not device<->device for many backends
Big difference for ffmpeg especially (but I imagine for curl too): it's not just one guy in Nebraska. Seems to have a very healthy community of devs involved in it.