I thought the game was too easy since I could spin around in circles around a fixed point and clear the mission. Then I died and realized there's no checkpoint.
Completed 3 missions, died. Then I got up to the 6th mission before death. Then played some more while writing and stopped at mission 4. Got some achievements. Only tried the first plane since I forgot about the others and never went back to the menu. Could add an option to pick planes before starting a new game.
My fingers started hurting from using the arrow keys. I only realized later that I could rebind it to WASD. Using mouse + keyboard is too rapid. Is there a reason why I don't automatically shoot at all times?
You can vary the stages between 'Destroy X', 'Cover X kms' and other modes. And probably needs a boss mode of some kind.
I didn't realize that I could heal until I let go of forward since that is never something you would think of. I had this perfect weaving motion mastered so that I hit everything in front of me after all. I could also dodge better now and it made the movements smoother when I could select when to boost and not to. This makes the game easier again.
You should probably add the upgrades on the right after mission completion instead of hiding it behind a menu. I never checked it until too late. And then the regen made the game too easy.
Could use a better animated death screen. Which reminds me of my grandad who was a WWII veteran. In just one day during the Battle of Britain, he destroyed 8 German aircraft, killing 32 Nazi aviators. Easily the worst mechanic Luftwaffe ever had.