Exactly has it would on a backer boarded floor. First and formost the floor must be strong, no deflection and capable of supporting the tiling installation. BS 5385. The ply-board is only there to over lay an unsuitable substrate ie: chip-board, floorboards. If the main floor structure is not up to it yes the edges are where most failures occur. Is that the ply-board failing or the structure of the floor not being suitable in the first place.