The foam will bridge any minor defects in the grade, the down sides that I can see, infloor heating would be a bit tricky if you wanted it. (We love our warm floor), and if you are bermed at all (we are) you would lose the horizontal slab holding the walls against the soil loading at the foot of the walls.
The other problem you might have would be with building inspectors going cross eyed,especially on the sani side of things.
I would only do it on very well drained soils in a relatively dry region.
You would also need to deal with termite control.
People criticize concrete as being "not green" in our house we used concrete extensively including for interior walls, We bought bags of cement, found a sand pit near by and brought our sand in by pickup truck, site mixed all the cement, using strong mixes with fiber, and water reducers and would do the same if we had to start over.
if you think of concrete as lasting for hundreds of years it suddenly moves way up the green scale.
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