Pay-to-win system. After reaching certain level, even the single player missions can not be completed without paying actual money. PvP is also broken. Instead of being matched against actual players in arena mode, you will end up playing against fake bots impersonating players, and they will be able to score ridiculously high amount of points with weaker birds, once again forcing you to pay money for gems in order to improve bird level or use options such as "retry room" or "shuffle cards".