Last time I tried to encode a video from H.264 to HEVC (hw-accelerated) on Linux it was such a pain to get to work that I eventually gave up and simply accepted the performance hit. I'm sorry, but I'm not gonna recompile ffmpeg so that it works on my machine. Considering that most RPi-users probably use a Linux-based OS this is IMHO a sensible decision.
Well it's not that uncommon to use a version of ffmpeg with more features enabled for a specific purpose. For instance my jellyfin server uses jellyfin-ffmpeg[0] to do hardware acceleration, even on my Pi.
For hardware acceleration it's probably easier to use gstreamer, depending on what device you are using to decode. But then you have a whole new problem.
