Well, given that the quality of hardware encoding is not (yet) as good as software, there are several advantages in using Rigaya encoders over FFMPEG ones (that's why many - if not all - encoding-suites are adopting them):
- many specifically-optimized filters (nnedi, for example);
- better control of hw encoder;
- deeper documented;
- more fine tuned.
That said, I'm personally very interested in using (Intel) hardware acceleration for filters and/or AVC "lossless" encoding.