I must commend the developers for creating a highly addictive and captivating game.
However, they have done an excellent job of demonstrating the extent to which game balance can be compromised. By enabling players to purchase units and enhance them with real money, they enable these players to dominate the competition.
This creates a highly unfair advantage for those who are willing to spend $100 to significantly increase their damage output, resulting in a game that heavily favors those who pay to win, since each card gets stronger the more you level them.