I have done only one 'floater'...A large kitchen 600 x 300 slate with the afore mention Fastflex and grouted with GT1 over glued and screwed down 8x4 WBP. The customer was aware of the possible failure and was also aware off what prep was req as his Dutch Father in law had had a floater of his tiled previously.
Like others I am not aware that this has floor has failed,and I'm not hard to contact as we pass each other most mornings going to work in opposite directions.
SAying that....I went to look at a small repair job following a water leak this afternoon. Turns out 300x200 slate floor tiled straight on to a floating chipboard floor with very weak sand and cement mortar,result.....total failure of the tiles in 2 rooms.
So it can be done correctly,some of the time!