Installing the Granite over the tub often creates a weak point in either the strength of the tub deck or the waterproofing material. It is not uncommon to find slab installers using nothing more than construction adhesive to install these slabs.
We have been recommending that the underside of the drop in tub be stiffened up and then a water proofing system used over top of the tub deck. On my last we lightly scratch the top of the tub deck and used a product called Ardex 8+9 over top. Some tubs have a tile flange you can purchase for waterproofing a drop in tub for a tub equipped with a shower. Either approach with an undermount look will require some steps outside of any recommended approach outlined by the TCNA.