Well, aesthetically there's nothing wrong the backs in and of themselves...but is there something you want them to imply, or not imply?
The majors are very lively, all together. Some I'm seeing for the first time--I really like the Hanged Man. I note that although you like to have "vertical" elements bending in toward the top, you have chosen to keep the verticals vertical in some cards--denoting moral rectitude in the Heirophant, upward growth toward the Sun, and the uprightness of Justice?