1/4" Hardibacker cement fiber board for shower walls
1/4" hardibacker alone is an unsuitable substrate for shower wall tile. If it were attached to properly waterproofed drywall, plywood, or plaster it would be fine. Think about it, one wayward elbow, knee, or hip bump and you may get flexion which would cause the grout (or tile) to crack.


  what is the preferred method for laying tile on concrete slab floor?


It depends on the application. Concrete is porous, so for water applications (like a shower floor) there are specific established methods for waterproofing, sloping, and draining. 

For dry area applications, concrete is the perfect tile substrate as long as it is clean and dry. Just apply thinset mortar using the application technique recommended by the mortar (and/or tile) manufacturer.