Ozai blows (no pun intended) as a character and personalty. But the reason I'm willing to let him slide (despite him being so BORING), is that he works fine as a goal for the rest of the cast in the show. Everyone else is developed so well in the show. And in the case of Azula, Iroh, and Zuko who have a direct connection to Ozai, they're shaped by his actions and the consequences are carried throughout the show (Zuko wanting his love, Iroh being betrayed by his brother, and Azula being molded into the monster that her father approves of). Ozai works as a destination to the journey's end. He is the goal of the show, essentially. Everything bad that the Fire Nation does is through him, or continued from his father.
The thing about Kuvira is that they wanted to bring her in, say she's some sort of evil person, then say she's complicated, then vice versa, without anyone having any real connection to her. The Krew's lives weren't drastically shaped by her (except Bolin). Korra really doesn't have a personal beef with her that isn't her obligation to instill BALANCE in the world, Mako and Asami are doing their own thing regardless of whatever Kuvira was doing the past three years, and Bolin was more than happy to help her out. Even when he figured out she was nuts, he wasn't particularly hurt nor was the revelation anything bigger than trying to convince Opal he wasn't a bad dude. Even then, there wasn't as much weight to it as opposed to when Zuko finally figured out his dad was bad (it rhymes) and made the decision to tell him off and join the Gaang. Bataar Jr. had more of a realization then that, problem is he's useless right after. Hiroshi did more than him when it came to it. If there was anyone who should've had more focus, it should've been Suyin. Develop THAT. We're constantly being informed of their past relationship, but not even in Book 3 was it hinted at.
Ozai was never painted throughout the entire series as a guy with complexities. He was just a mad, power-hungry villain and they played it out to a T and didn't pretend otherwise. Not. One. Bit. Kuvira was She-Hitler until they said she was an orphan and just wanted to protect her people. Suddenly they wanted to switch gears on who she really was and it didn't fit the bill. Lack of a real background on Kuvira and her relationship to the main players is what killed her. If Kuvira was a She-Hitler played straight that would've been fine if the rest of the cast had their destinies riding on the line of her defeat. We didn't have that. That's why the finale was really a perfect analogy to the entire show: superficial. On the outside it does cool things and looks fantastic, but there's nothing underneath that propels it and makes you care for these people.
But yeah, as a plot device Ozai works better than Kuvira. Both are bad characters, but at least Ozai was utilized properly.