You say that as though Unalaq was only real problem the show had. He was just the most glaring. All the villains were shitty. Yes, all of them. Amon, Red Lotus, Kuvira all started out interesting and had some great potential, but in the end they all just fizzled out. Unalaq just gets the most shit because he was shit the whole way through instead of just at the end (or the whole second half in Kuvira's case).
Though I'd argue that the series most glaring flaw wasn't it's villains or even Unulaq (I don't know how to spell his name and I don't care enough to look), it was Korra's "team".
Everyone that was with Aang had an actual reason to be with him. It felt organic the way they came together and grew as a group. Plus the goal of stopping the war was something they all shared for obvious reasons. The only time Korra's group ever really had a mutual goal or reason to stick together was the Amon thing. But even at that, the threat to Bolin and Mako was just a general one, because they were benders. There was never anything personal about it like with Katara and Sokka.
Hell, in season two, Korra's group is of so little importance to the show that they spend the first half of that season involved in their own separate plot line that's completely irrelevant to the rest of the story. It was a classic case of "giving them something to do". Which is usually a pretty telling sign that those characters aren't actually necessary to the story.
Mako is boring. Bolin is a useless manchild that couldn't even succeed as comic relief. And Asami's purpose can be broken down as such: 30% forced drama love triangle bullshit, 40% barely distinguishable from a background character, 20% actual competency and getting shit done, 10% now we're lesbians!