The game is inherently stat-checky and the upgrades pretty uninteresting because of it. Whenever you hit an enemy you either kill them or take and do damage. The early stage of the development also results in some additional issues like being able to get stuck early on.

HELP FOR GETTING STUCK: Go for survivability and raw damage (the Guidance upgrade) until you defeat the first boss. If you buy a significant amount of "hit more" upgrades like the extra bounces one or the increase enemy spawn upgrade, you will die and can get stuck without ever being able to even see the boss anymore.