What I hate about the mini game is the randomness. There's NO player control over what sort of gems will be generated. Need a blue gem? Here are a bumch of useless red and green. Need a level 1 red gem? Here are a bunch level 2, 3 and 4 red gems.
Then there's the imbalance in the gems on the board. Example:
View attachment 24799
The two unlocked gems were red, so ignore them. Look at the Green gems instead. How many green gems give a lower key part? TWO! Every other green gem gives upper key part. So best case scenario I can get 2x3 keys, plus 1 more for having 4 green gems with half a key on the board.
The Blue gems are even worse: ONE upper key part, everything else is lower key parts.
Only the reds are somewhat balanced.
In my main city I am down to just 200 energy left and my progress is 283. Yes, uust ONE Grand Prize so far (not counting the free one at the start). If I want to get more Grand Prizes, the best way seems to be to open one golden chest per day, the one that gives the most progress (because the first oen gives double the reward). Which means I will be ignoring the game for most of the event (until I need more keys for chests).
Overall it is not a bad mini game (once you figure out how things work), but the randomness ruins it. Skill is meaningless if you are at the mercy of RNG.