[Unbalanced rewards]

Agreed. Pretty much all about the gems and trashed(or elliminated) all the chances to get anything else.

[Frustration]

Love the game. But, you get to a certain point (different point for everyone I assume depending on whether you pay or not) and all you do is lose. Most losses anywhere give no rewards at all. So, get frustrated and reluctant to waste the time for no reward. Thinking about quitting.