This is super fun and addicting. But some of the mechanics seem poorly balanced in a way that makes the whole game feel less and less interesting the more you progress.
The upgrades on the skill tree seem like they'd be incredibly useful in the early game, and they look enticing, but because skill points are so hard to obtain, it's impossible to unlock these skills until you're already so powerful that they've become obsolete. For how long it takes to get through a whole gauntlet of bosses just for one skill point, the reward is incredibly underwhelming and it doesn't feel worth it.
The powerups are pretty cool and satisfying at the beginning, but after a few hours of gameplay, you quickly pass the point where they become useless. It would be great if you could upgrade these in some way, so they keep up with your advancing power level and always feel impactful.
The balls themselves also become less interesting as you go on. It doesn't take long to achieve such a high DPS that you're chewing through levels in less than a second, at which point the individual abilities of each ball (like the splash damage of the plasma ball, or the poison of the poison ball) just don't do anything anymore. I think either the ball progression needs to be slowed down significantly, or levels need to get exponentially harder the more you advance, so they don't become trivialized.
Overall, the progression feels extremely satisfying in the beginning, but that enjoyment quickly tapers off as the progression becomes less and less rewarding and you don't have anything impactful to achieve.