What can go wrong with connecting a mixer with a soundcard, assuming you have the right cables? It's not that there's going to be any hardware compatibity problems or drivers issues just because you hook a mixer on to a soundcard.
If the mixer has the I/O that you're looking for and if the soundcard has the I/O that you're looking for and if you have a clue what their routing capabilities are, what do you expect to go wrong? It's just like hooking the mixer up to the PCI version of the audiophile or any other similar soundcard. You should expect more issues getting the USB audiophile to work with the computer but that's a different ballgame.
It's all about interfacing analog audio signals. Nothing more, nothing less.