Whoa, I missed something major here. Mastic is not pre-mixed grout. The mastic I am referring to is an adhesive that you use to install tile, it is not grout. I think the brand my old company is using now is Chapco, btw. If that stuff failed, the company should honor your re-install costs if your surface was proper.

Bone, I meant installing the actual shower tiles on greenboard. Destined for failure. It is perfectly fine for kitchen backsplashes and wall tile in the head.