Yes, you DID make it too hard with that spawner ship. It easily blanketed the screen with the smaller ships, and if you 'accidentally' killed all the bad guys shooting at your city, you're left helpless to watch them easily kill your city. The description for the machine gun and flak buildings neglected to tell you that all it does is give you a powerup, replacing your standard weapon with a gun of that effect. The game would have been alright, if it wasn't for that oversight.
Tone down the spawn rate on that boss ship, and be a little bit clearer with the descriptions please. As well, state that the sell button replaces all spent cash on the building sold, and tell the player what kind of currency you're using to build with.