From what I've heard the first gen QX56 had pretty bad reliability. Especially the 2004-2007 model right before they did a mid year face lift on it.
One big contribution to their poor build and reliability was the fact they were built in America at the Nissan Canton, Mississippi plant with Nissan Armada and Nissan Quest. Everything that came out of that place was nothing but problems. That's why they have shifted production of the QX56 to Japan for better quality. Actually the QX56 use to be the only Infiniti built in America, and what a coincidence that it was the only Infiniti with the worse quality.
Do a search on google and you'll see reviews people have made talking about thousands of dollars they've spent in repairs from water leaks, crapped out engine computers, controls not functioning, etc etc.
