the problem you have is when moisture comes trhrough. Grout and tiles are naturally porous.
And I am sure many tilers have seen where customers have not kept up to the maint. of their bathrooms. I.e. How many customers know how to seal a bath corectly before you tell them!
Plasterboard does turn to mush, Plywood and chipboard can warp and swell causing tiles to pop off. I would recommend any of the system mentioned.
Hardiebacker is avaliable in Topps and I would recommend getting a trade card.
However it is stocked in many other places.
The main thing id do it right the first time and it wont come back and get you!
How many tilers get great work on recomendations, what would happen if one job went wrong and the word got out?