With USB connected DACs the most signification factor is that DAC itself should use an asynchronous USB connection. This way the clock is managed in the DAC itself and not reliant on the PC.
After that we are back to generated EMI (electrical noise) again, but of course this time the DAC is directly connected, sometimes even powered from the PC, so there is a greater potential for contamination.
There are ways to help mitigate this, such as building a dedicated audio PC and fine tuning the OS processes that run on it to reduce the loading. This is quite a detailed, sometimes controversial. I’d recommend having a read over on the Computer Audiophile site to research that topic.
Personally I believe that networked connected DACs (with good EMI shielding at the DAC end) is the way to go and I believe the industry is trending this way as well.
BTW SPDIF out of the PC is the worst way to connect up digital audio as the signal is clocked by the PC’s hardware; which often results in a lot of digital jitter.