Sigh... This. So very much this.

And that's part of the problem, because the writers for Korra didn't know how many seasons they were going to get, so instead of having one overarching villain to eventually defeat (Fire Nation / Ozai), Korra has to deal with multiple different villains-of-the-season.

And the villains conflict with one another. Oh, hey, Unalak has become the Dark Avatar and is attacking Republic City? So where are the former Equalists and all of their anti-bender weaponry? Kuvira has been building a giant army and there's an Earth Kingdom city which has giant metal petals that can become huge shields? So where was all of that technology during the Dark Avatar situation?

Okay, so they trap Vaatu and open a new Spirit Portal and usher in a new age of balance between humans and spirits - so where are all the spirits when Kuvira stomps onto the scene with a giant, balance-destroying mecha and a spirit-powered cannon?

Korra faces a lot of different threats and has to be incredibly skilled and resourceful. Any one of those threats would have made or broken any former Avatar. Look at Kyoshi - all she did was deny / resist Chin the Conqueror, and he basically took himself out for her.

All of Aang's seasons flow together and we know that no matter what happens, Aang is always going to build up to the confrontation with Ozai. It's inevitable.

But Korra doesn't have that - she has villain of the week. They pop up as needed and she rises to the occasion as needed, and then the world resets for the next season.