Quite a lot of fun. I think balance is initially okay, certainly winnable, if anything gets easier after a few levels once you've upgraded a bit - I think ease of upgrades outweighs increasing difficulty of enemies. Actually I found I couldn't go much beyond level 20 due to being way too overpowered - once you have a couple balls whizzing around at speeds of 200+ then you're just going to smash the level easily. And by level 28ish I had upgraded so much that the balls just got into a weird feedback loop where each bounce off the walls or enemies (I wasn't doing anything) made them faster and faster ... eventually they were going at speeds of several thousand and then the game just froze. Or sometimes the balls seem to go so fast (once their speed is above 10,000 or so) that they leave the game altogether, but it freezes soon after that anyway...
Typo alert: "your safe" at the end should say "you're safe"!
EDIT: might also want to have a time bonus for fast level completion that's visibly ticking down to put some pressure on the player to take the initiative. The first two levels can be won just by standing on a high platform where you're unreachable, and letting the enemies walk onto the ball. In fact knockback is the only way you can lose health (because you can accidentally hit them up to the height of your platform, which is the only way they can touch you!) so you're best to just let the ball float around and be utterly passive. A time bonus would encourage some risk-taking and less boring gameplay!
EDIT2: think the fatal bug is in the "reduce deceleration" upgrades, since with enough of them, the ball seems to actively accelerate on its own, which quickly crashes the game! Rather than a proportional or constant increase to this bonus, perhaps you should make each extra upgrade have a lower effect than the previous one, so that overall their effect starts to level off...