It does seem occasionally some of the elements don't shoot like if I place a laser as the stuff is coming down, it just refuses to shoot. Unless a new round starts and it goes about its thing then. And the upgrade system was a little strange to understand I didn't see an impact myself when upgrading them per say.
For the balance I think its quite in a good area and can be handled the boss fight it did seem might become a little difficult, other than that seems normal