I concur with what everyone has said so far. The USB gen 2 card is excellent in the Bifrost Uber. I cannot hear any difference between USB, coax, or optical. In lesser DACs, I could hear slight differences over USB.
Also, unless you need balanced, the Bifrost is a better a value. Some reviewers, like Purrin, has said the Gungnir is slightly better at unbalanced than the Bifrost, but it's not very much. If you might eventually go balanced in the future, go ahead and get the Gungnir. If you will always be unbalanced, go with the Bifrost. They are that close in quality while both running unbalanced. The Gungnir takes it to another level over balanced.