Neither Eb, nor Harry could touch Raith with magic. It was like it just slipped off of him. Maggie on the other hand cut his privates of (in a sense). She hit him where it hurts most. His ability to have pleasure, both physical, and his love of controlling women. She destroyed him. Now either she targeted him in a different way than Eb hadn't thought of, had help (from Leah), or Raith didn't get that magical protection until after he got neutered. He may have made a deal with a certain Outsider, demon, or Drakul for some sort of protection.. The red gem he had maybe?
So anyways do you think he's had that protection for a long time, and Maggie got him regardless, or do you think it was after she hit him that he got it? Maybe Nemesis took advantage of his weakness, and fear to get a foothold in reality.