I've worked on houses where someone installed tile over partical board, hardwood floors, 15 lb. roof felt, loun, peel and stick tile.
Non of which are exceptable underlayment.
The customer kept asking me to remove the cracked grout and redo it. You try and explain to them why it's failing and all of them said they could not afford to redo the tile. I make them sign a paper saying I told them it would not work and they still had me do it. Everyone of them failed again and expected me to go back and redo it for free, so I just stopped doing it at all. Tile is a one shot deal and 90% of it is in the prep work.
We had one that we removed all of the tile with nothing more then a flat shovel, no wonder it was cracking.