I liked the wave system and the way upgrades work it fits the theme and adds a fun amount of strategy to the game. I think the reason people told you the game is boring is because the players actions feel empty. You don't give the player enough feedback for their actions. You can try to add some sound effects, particles, slow the game down for a moment when hitting an enemy, and see if it improves the game feel. Your game has good gameplay it just needs more juice and to work on the balance.