So the black screen in HW-mode is something related to the fact that some graphics done using CPU? But what exactly done by CPU? Others said there's troubles with VU0. So what to believe?
And how Sony solved the problem when they released Primal in PSN?
GSDX HW is very limited to certain things, Software is much more capable as it's much easier to do stuff (you aren't limited to tricks in OpenGL, DX etc).
That's probably just it.
Of course it can surely be done in HW, just not now or with how it's made.
That's.. not specific answer to a specific question. Most of the games with certain glitches in HW is all about what you said. Even wiki has some info about these glitches. But nothing specific on Primal. Only 'huge slowdowns' and inability to play in HW and nothing else. (Yet I remember earlier builds were able to output in HW with the same super slow result)