Myself I would not use foam against earth in any situation I can think of. My exceptions would be making a homebuilt cooler with sprayfoam - very efficient. A local peach and plum guy brings his stock from colorado and georgia up here to south dakota in a 1/2 ton with topper, interior of topper sprayfoamed with bubble foil baffles, inverter, mini ac into custom slide out, coolbot. I would also consider foam as a great way to insulate and utilize a grainbin for a home or shop, but in that situation (interior foam spray and then stucco inside a reclaimed bin) a whole house air exchange system would be desirable in my climate.
I think that there are even many other potential odd materials that will make good cob. Yak hair, yucca fiber, lambsquarter fiber, long strip pounded bark (oak?), palm leaves, canes or reeds, and pretty much any other fiber if you were somehow in a survival or extreme remote situation (jungle, mongolian foothills, pacific island?). A material I have used was whole entire western red cedars stacked above a no-straw cob mix, laid on gravel substrate. Stacked more cob on top of the brush. Ag building so why not try it. Maybe after a while I will dig it up with the skidder and see what transpired (they were still a bit, um, green).