Here is a review article about the Bifrost (regular) from Nov 2012:
It’s a good-sounding, reliable, solid implementation of USB. But that’s like saying, “Well, its a very nice meal, given that the chef could only work with McDonald’s hamburgers.”
Wait. Are you saying USB is crap?
We’re saying we put a ton of time into our USB implementation, but, to our ears, USB still doesn’t quite offer the performance of SPDIF. And we can even get into shades of gray on SPDIF too: consider Mike Moffat’s AT&T ST-optical interfaces and Sumo’s Axiom/Theorem transport and D/A, which had a separate low-jitter master clock connection from the transport.
This section of the FAQ seems to have been taken out, so perhaps the gen 2 USB does sound a little better than the gen 1. Still, if your source already has SPDIF, why not use it and save the $100 - $150?