Azdel has a number of benefits...
It is impervious to moisture, will not absorb drop. It its far lighter and stronger than Luan much less the cheap plywood many manufacturers still use. Also it has a higher "R" value than the other materials. When properly installed using the vacuum seal construction techniques it will be sandwiched between the fiberglass and foam insulation and the entire wall becomes one solid, highly water resistant structure. Lance was one of the early pioneers to utilize this technology along with Nexus.
Some of the higher end Winnebago lines are now going this route as well. Using Azdel is time consuming and expensive but it produces a superior finished product.
Here are couple of shots I took back in June of 2014 while touring the Nexus factor. The first {IIRC} is part of the vacuum press and the second is of a roof under construction. Nexus uses low alloy steel for all of their framing instead of aluminum or wood for greater strength and utilizes one piece fiberglass roofs and end caps:
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That is our coach in the background behind the roof section. I was having them install the spare tie mount under the rear of the coach {at a reasonable $75 an hour}.
:)