USB headsets require much more CPU overhead than a good sound board does. The difference is significant. Anything requiring a lot of data, such as sound or networking, is really not suited for the USB bus. It does compromise CPU performance. No matter what kind of glowing recommendation someone gives a USB sound device, that is just the way it works. It will use up more CPU power than a dedicated sound device would.
The secondary effects are due to how Windows works. If you also have an onboard or dedicated sound card and it is enabled, Windows will poll that device each time a sound request is made to play a sound. This can have other negative impacts such as stability issues, sound stutters or pauses, or generally garbled sound. It does not always happen, but it is possible for it to happen.