While I don't know all the technical merits, I know that the USB DACs will exhibit similar jitter problems. This is because USB audio is still streamed over, it is not the same type of transfer as say a USB hard drive has. There's more than one thread relating to this, and I've read an article about the issues with USB audio. I'll see if I can't find that USB article for you, its an intersting read. : )
And the advantage I guess is there is one less piece of equipment in the signal path. Instead of going from S/PDIF from Sound Card---->DAC----->Stereo, your just go Sound Card(or USB DAC)----->Stereo. Although, of course, this does not guarantee that the latter combo will produce better sound.