An ITX rig with a Nano should be properly small, just fitting the ridiculous amount of hose that comes on two AIO's (especially when one is the FuryX) would look pretty bad. Nobody to date has put a pump of comparable quality to a DDC/D5 in an AIO, you can definitely get quieter. Then obviously their is efficiency gains from running a single loop with the same radiator area rather than dual loops. People would even advise against that left right and centre in a full ATX rig. A custom loop would be superior from both an aesthetic and acoustic perspective. It would however be a lot more expensive, involve more risks and more skill. I can understand why more people couldn't justify doing it for those reasons.
JR