Foam has a bad rap about being cheap from the injection mold kits from the 80's. Shure Flite I think they where called? As for making the planes your self I think it's just a matter of getting all new equipment and acquiring new building skills. I thought about getting some foam cutting equipment myself but when I saw the price of wire cutters I gave up on it. I got some more wood working tools instead. I have to admit to feeling let down when I see "wood/foam construction" at the end of the description of a plane I like in a plans catalog. I just like working with wood. I think it's just working with wood gives you an old time craftsmen type feeling whereas foam in more of a modern mass-produced cold industrial feeling.
I don't like foam personally from making plugs for fiberglass molds out of it. The main thing was foam dust from sanding. That stuff gets everywhere and sticks like mad because of it's static charge. I tried to vacuum it up and that crap just jumped on the vacuum nozzle and hung on to it.
Clay Ramskill has a very interesting article on it.
ANALYZING THE FOAM WING