I don't know about the rewards being completely useless, I got block on Olaf and on Singed I got magic defense. Though on Morgana I also got magic attack.

There is a button to reset stats for sanctify, however I am afraid to use it as I don't know if it takes you back down to mortal or not, or if you will need another card to get a new stat. There is very little explanation anywhere on this function of the game and the cards are too expensive for me to be the one to experiment with them to find an answer.