Yeah, her motivation is basically the archetypical, Chinese emperor, the world must be one and unified (see: Yimou Zhang's Hero among many other stories). She fits the pattern of the show, though. Pretty much all the major villain's true motivations have been a big late season reveals. This has been about Korra and the bad guys are just her boogie-men for most of the show.

The difference for me with Kuvera, and the reason why she feels flatter as a character--though she's still exciting--is that given what we know about her so far without the big reveal (if there is going to be one), she is so unsympathetic. Amon was nasty, but you could ride with his fight for equality. Unalaq was batshit but his desire to unite the people of the water tribes felt genuine and familial (even as he used and abused his own family). And Zaheer's charisma, eloquence righteousness, and nobility made you not just understand him, but want to follow him (100% the best of Korra's adversaries). Kuvera, though, just seems like a cruel despot. Her desire to see the Earth Empire unified doesn't feel righteous or for the people. It seems selfish.

Perhaps its because I've seen the unify China tale played out in a way where I totally understood the villainous means to a righteous end, that I can't help but see Kuvera as lacking by comparison, especially when given a whole season to develop her.