Korra never matured as much as a person the same way Aang had to, despite facing more pressing matters that all put her existence even as a concept into question. The only maturity she gains throughout the entirety of the series is one of interpersonal relationships, not just with her companions but also with her enemies - which Aang also achieved. Maturity of character and other aspects are still nowhere in sight 3 seasons in, because she does believe in that "being the avatar" is basically all that she is defined by and all that she should be defined by.
She never loses her massive temper or her headstrong attitude and since every major villain is handled in a bombastic finale that ultimately boils down to nothing but a "good vs evil" fight, when the show just went the extra mile a couple episodes beforehand to establish them being more layered in shades of gray than the Avatar series has done before is just insulting - why even try to be ambiguos with your villains and have them carry emphatic causes when you just want to villify them in the next episode? Why even try to build up different philosophical concepts that each and every one Villain embodies when it's all just resolved on a physical level without even so much as to challenge the philosophical level with more than a handful of throwaway lines? This is the equivalent of someone trying to be profound by taking a stack of books on philosophy, dropping them in front of your feet and says "here you go, that's yer lot. Go figure out the rest from there."
It's basically a mixture of bad writing and it being a massive missed opportunity.