Ruthless
I think this is the most ruthless game for monetization I have ever seen.
As you progress you encounter endless levels that take fare more energy than you can ever hope to have in your collection, and even competing them takes 5 or 6 run throughs using 70 or more energy every time. as you progress you value gold more than anything else, that becomes harder and harder to obtain without spending money
Ir really is rather sad how it has become and how obviously much money people spend on this.