The 2001 Ford Expedition had an aluminum hood. I found out about that when it was hail damaged, and the hood had to be replaced, rather than repaired.
Once dented, aluminum has too much of a memory for the dent to be pushed out.
Our 2005 Expedition is the same as the 2001. The rear lift-gate is a composite material. (I think)
As far as I have heard, aluminum body panels are not as durable as their steel counter parts because of the relatively low resistance to rupture (think about how hard it is to puncture a soda can verus a steel can of tomatos). Any kinda of large rock hitting the front end would put a hole through the body panel. Now, almost all SUV's have had aluminum tailgates (lift gates) in the back since 2000. For a while, I know there were also issues in applying paint and clear coat to aluminum because of the bonding process, but I think those are mostly resolved. Now the major issues would be getting everybody to retool to form aluminum panels instead of steel (and the steel industry would not be happy).
The Mitsubishi EVO has aluminum fenders and hood, for weight reasons. Personaly, i wouldnt want alum parts, its too soft, and onces its dented, you have to replace it. Might save a few lbs, but not worth it in the long run for durablity and price.
let's talk about the ones we can afford! for the audis and other high ends, i believe only the panels are aluminum such as the hood, trunk, and some other body parts.
The A8 has an aluminum frame and body...the whole body. You are right in a way that there are some high end cars that have a piece here and there for weight. I use the A8 because I know for certain is ALL alluminum.