The worst I've seen is tiles fixed straight to untreated MDF in a shower, the bottom row swelled up like that Hedgehog that they had to pop.
Is the fall away from the drainOn this subject I had a builder call me yesterday to book me in for a wet room job, to tank it etc and tile. they have screeded the floor and put a drain and in 'made some fall'.
Thanks to everyone who replied. After much consideration I've decided to take no chances and go for a regular shower tray!