It all depends on how far you can suspend your disbeleif. A lot of the negitive reveiws I've seen say theese things. My responce in italics.

1: Charecter development is terrible.

Not every MG/YA book has to end with the protaginest having an epifany/revelation and suddenly being the best person around. Throughout the series, everybody grows and matures, and some self-aware comments are made. I love the way it was done, where you see from different character's perspectives at times and learn something about them. For exmple, in book 3 we learn Charlene has 3 cats. Those interesting factoids, at least for me, are wondreful for getting to know characters.   

2: The mentor Wayne is terrible.
It's like Dumbledore. He uses the Socratic method. 

3: Finn is a terrible hero. 

I like Finn as a main protaginist. He goes through relatable strugles like the impact of fame on school, friendships, and family. Not to spoil anything, but also he is a really great leader for several reasons. 

4: I'm not a Disney nerd, so I don't get it.
Understandable, since the entire book series is about Disney. But for Disney ners (like me) who can keep up with the story, it's plenty fun.

5: Malefecent is wrongly portrayed.
No spoilers, but the way I see it, you're SUPPOSED to see her as different than the aniated version from "Sleeping Beauty". Think of her more as how the was portrayed in Disney Channel's "Desendents", if that helps.

6: the girls don't do much and it's a bit sexist in that way.
Maybe slightly in the first book, but they arn't without their strong moments. It gets better as the series progresses. There are even some really great Momma Bear momments with the moms. 

I love this series, and though there admitedly some recognisable flaws, I choose to suspend my disbelief for this fictional story, and enjoy it.