[Saurreg]

I am under the impression that Hurin was released from bondage with his death. I also believe that the curse of Morgoth only extended to the affecting the life of the one being cursed and nothing more. The thought that Hurin might became a wraith never crossed my mind and I do not think it was possible under circumstances.

Hurin's fate was forcifully altered by the dark menace of Morgoth and as such his ka was afflicted by a force majeure; one of nature that was beyond Hurin's innate ability to resist. The nine ring wraiths on the other hand fell into temptation and accepted the power of the rings which was essentially infused by the essence of Sauron. With that I believe those beings' spiritual bodies were contaminated by the extented presence of Sauron who was then able to slowly influence their thoughts and feelings and ultimately control them - all because they had unwittingly allowed him influence over their very being through manipulated invitation (i.e the lesser ring of power). Over the ages, all things must change and I am under the opinion that the original bodies of the nine had withered away but so strong was the presence of Sauron within them that his power was able to sustain them and give them a supernatural consciousness and bound them to Middle-Earth. What was left of their spiritual body might have evolved/mutated beyond the original Ka form and have become an off-shot entity that is controlled and tied to the fate of the puppet-master itself. Which is incidently why I think the remaining eight perished as soon as the Ring; the source of Sauron's concentrated power was destroyed.

The key difference between Hurin and the wraiths were that the latter had his fate forcefully altered but he might not have broken down mentally and spiritually. The latters might not have accepted Sauron directly, but by wielding the lesser rings of power, they let their guard down, gave in to temptation and accepted the control of sauron indirectly.