YMMV / Sky Force Reloaded

• Difficulty Spike: It depends on your gameplay style, but Stage 13 is very likely to be more difficult than the preceding stages. As the final stage, it's designed to be challenging even for a fully upgraded plane. This means that unlike every previous stage in the game, you can't trivialize it by grinding for upgrades in previous levels.
• That One Level: Stage 5, which disables your weapons at the beginning of the level, forcing you to rely entirely on your dodging skills. You can find a permanent card which disables this, though it replaces it with a homing missile that will disable your weapons anyway if you ever let it hit you. It's even lampshaded by one of the loading tips, sarcastically calling it "everyone's favorite" stage.