While the sophisticated engineers among us will argue that USB is also frought with problems, there is no doubt that SPDIF is second only to the transport itself in introducing 'stuff' into the signal path. More specifically I should say the implementation of the SPDIF interface - with special emphasis on the bad boy cable...
In a perfect world, the DAC would take in the USB signal and convert it to I2S instead of SPDIF. But even if it simply converts it to SPDIF before sending it to the DAC chip, it at least eliminates the output>cable>input problems.
All that said, while you hear it??? YMMV