A common flaw:
If there is one fault that is shared among nearly all leaking bathrooms, it is the porosity of tile beds. Water from a bathroom soaks through porous tile grout, in between tiles and into the tile bed beneath. (The tile bed is typically 30-40mm thick.) The tile bed is a simple mix of sand and cement, but still behaves much like sand at the beach, happily absorbing water like a giant sponge beneath the tiles.
The wicking effect of this 'sponge' through capillary action saturates the entire floor under the tiles. Before long, the moisture will expose any weak spot in the waterproof membrane, and that could be something as tiny as a pin hole from accidental damage during construction.
Any waterproof membrane has to work hard under constant saturation, and any compromise in that membrane can mean problems.
Mixing a small amount of Efflock makes grout and tile beds hydrophobic to repel water absorption. This ensures a healthier bathroom with a primary barrier against leaks.