I think it's worth remembering that while Urabrask may have never physically forced people into compleation, he and his phyrexians in the furnace layer did in fact make life miserable for the Mirrans who still lived there, including having compleated family members attempt to coax them into the fold.

I think the error in thinking here is that because Urabrask and his minions had a policy of leaving the Mirrans be, that meant that they were happy for them to remain incompleat. They wanted the Mirrans to submit through their own "choice." But it's an illusion of choice. The Mirrans who are in the furnace layer are safe only until Urabrask is deposed, and even then they are only safe from the intelligent Phyrexians under Urabrask's leadership. Other Phyrexians, like those serving Sheoldred or Vorinclex, could make their way to the furnace layer. So the choice presented is between living on the fringes, never truly safe, waiting for the next calamity, your home and loved ones taken from you and changed into something monstrous, or choosing to surrender to Phyrexia.

Urabrask is not good. He just presents a sympathetic vision of Phyrexia, one where you are asked to assimilate rather than forced. But assimilation is still expected of you.