I thought BAL's reply was due to a legal issue possibly. If they give advice on tiling a ceiling they might then be sued if it goes pear-shaped as their adhesive might be blamed.
My tutor said movement in the house might cause a tile to become dislodged. In that regards, the shower room in the house in question is upstairs and there shouldn't be anything to cause vibrations from above.
I have told the clients that if they wanted to tile the ceiling they would have to fix some backer board first. I did emphasize the need for strength and stability of the substrate.