You're absolutely right. I just didn't have the time to do balancing or a Boss for the ending :( Currently enemies are generated randomly based on some values for each wave, so sometimes it's super easy, sometimes its way too much. This should be solved with some proper level designing :)
The art/sound is not mine, it is from an old dos game called Tyrian 2000, which is awesome and I highly recommend you try it out, if you like plane scrolling shooters.