While PC is fun, lightweight, fire-retardant (it dissolves rather than inflames), it is also very absorbant, loves to drink up the moisture. Unless the permies/builders have come up with an affordable, 100% guaranteed against absorbancy mixture, I would not use it for your support walls, especially not underground.
But, it does make for a nice insulating plaster to finish out your home.
I hope someone with deeper research with newest technology (resins, etc) will correct me and give you the green flag. I'd be interested in learning about the 100% guaranteed advances made too.