I’m having trouble 3-starring (or even just beating) level 10 most days, but so far I can always 3-star level 8, and the only worthwhile reward I’m missing when I do so is the guaranteed cosmetic crate. I can’t say my opinion is set in stone after just a few days on this trial, but it feels to me like the balance is about right: it’s a challenge to complete the highest levels, but generally not impossible, and when it does prove too difficult, I don’t lose much by dropping down to a level I can beat.
One complaint I have is the choice of border for the bonus when first 3-starring level 10. Given how iconic Mickey Mouse is, it seems a bit unfair to restrict this border only to those who can complete a fairly difficult task. I think it would’ve been better if a new border had been created for people to use as a way to brag about their accomplishment.