On my 2014 Mac Mini (with a Haswell-era Intel chip) I've found that software transcoding gives much better picture quality than hardware transcoding. But the software transcoder starts to struggle a little bit when transcoding two streams at once, and it definitely can't do three at once.
It would be nice to have the option (maybe hidden behind a shift-click) to prefer the use of software transcoding but allow hardware as a fallback.
Ideally this would all happen dynamically, with the transcoder seamlessly switching streams to hardware whenever it starts falling behind, and then switching back to software once it's caught back up.
But I'd also be perfectly happy with the simple option to set a maximum number of concurrent software transcodes, forcing additional concurrent streams to use the hardware transcoder.