Be sure to check out the Alsa supported devices list:http://www.alsa-project.org/main/index.php/Matrix:Main
This is important, because almost all USB devices with 4 or more I/O channels are USB 2.0. Unlike USB 1.1 audio devices, which are class-compliant and will work with no fuss at all, USB 2.0 devices will need their own driver to work. Unfortunately, USB 2.0 audio devices with good Linux support can be hard to find.
Now if you're looking at a mixer with just 2 channels USB I/O, those should be USB 1.1, class compliant, so they'll work without any trouble.