Thuringwethil was just a gofer. She carried messages for Sauron, unquestioningly loyal to him. I seriously doubt she did anything evil worth noting, and that was done was by Sauron's orders. In fact, she even did some good (inadvertantly).
Ungoliant, however, was her own boss. Not only that, she was a machine of destruction. She consumed the Two Trees and the gems of the Noldor, causing the Noldor to revolt. Without her, Ea would be a much happier place.
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