I think the game design is really good and does its job well. This does include the aiming (which I've seen lots of complaints about), but in my opinion, maybe dragging to determine distance would be better. I can't remember if it was a drag in the original Balloons or if it used a system like this one.
The one game design change I'd maybe like to see is that you don't have to redo each level if you fail—for example, if you fail the third one. However, I completely see why it was done that way, and if the following issue were fixed, I don't think it'd be nearly as annoying.
The complaint I do have is that, although the game is designed well, the levels really aren't, especially on the harder difficulty. Of course, the levels are meant to be harder, but some of the changes make the levels super annoying rather than actually difficult. However, this isn't even the most annoying part.
The most annoying part is that, in most levels (especially the more difficult ones), you almost always have to get 100%. I understand why that's necessary sometimes, such as with puzzle levels, but that doesn't mean it isn't annoying. I feel like all these little frustrating things stack up and make the game significantly more annoying than it needs to be.
It's unfortunate that you're getting these comments, and I'm not just trying to pile on. Obviously, people—including myself—enjoy the game; it's just overly frustrating at times.