So, I will list some of the pros/cons about foam buildings.
So why would you buy foam buildings? Well, hand cut foam will be MUCH cheaper then casting in resin. Foam should be stronger and slightly cheaper then casting in plaster of paris (AFAIK, still have not tried it yet but I will). I have built foam bridges that can hold 30Lbs!

Hand crafted foam buildings will always be slightly different- bullet holes, mortar/panel lines, ect...
Complex, bulky designs are possible with foam, where as, plaster of paris/resin would be WAY too heavy.
Foam would be way cheaper then anything so far. I charge around $7-$10 when I make a building, so if I spend 2.5 hours on a tobruck bunker I can charge $15-$20 for it. I know (Like I said) resin will be probably twice as much given the amount needed to fill.
I harden the outer surface of my foam buildings with heat giving them a hard, solid feeling exterior.
THere will be other materials added too, not just foam! Well, maybe just all foam for bunkers but not for houses... I added balsa wood, plastics, ect. for final details. The foam just serves as the base.
Why you wouldn't buy foam buildings...
Foam buildings are light, you will not get the first impression of a quality feeling building. (For instance , pick up a Franklin mint B17 and pick up a Revel B17) But, I can always add some weight to them with steel balls or something like that embedded into the foam.
Foam buildings can get dented if not handle with some care in mind. So just be careful. (My foam buildings are surprisingly tougher then you think.
Foam does not like heat in excess of 160F so don't store them in the attic! lol, but that would be sorta common sense, no?

That's about all I can think of ATM.
Leave feed back and if you have questions ask them !!!!