It's good! I actually didn't feel that the game was as hard as others have been mentioning, but I do agree that there are some parts where it feels less than fair. A platform in the boss room would make dodging its machine gun feel much more fair.
I never found a reason to use the third mode! The first two have clear applications, but the third is missing an enemy to go with it. A flying enemy or one that hangs from the ceiling would make it be more useful! Actually, more varied enemies would be great, as the enemy variations don't seem to be very different from eachother.
Overall it's a nice start and could be expanded further. In a future version the weapons could be upgrades to give it a sense of progression, for example!