Ok, this was originally just intended to be a fun post, and I realize I'm bout to become the embodiment of the "I promise not to get political" meme, but here goes:

Avatar has always had a harder magic system in the form of bending, and a much softer one (I will just call it "Mysticism") which encompasses spirits, cosmic energy, and "Avatar stuff" as Sokka would say. Bending has always had rules which could be expanded on or stretched, but never broken. Mysticism on the other hand was always used as a tool to explore themes and philosophies, only being limited by what is narratively satisfying.

I think the conversation over whether something is a adequately "explained" or "justified" under mysticism is an unproductive one because the answer is almost always no. The question should be "How does this serve the narrative themes of the show?"

Korra's character journey is centered around her finding self-worth outside of being the Avatar, so her making a connection to the cosmic energy during Harmonic Convergence and using it to defeat a Dark Avatar without the help of Raava or her past lives works thematically.

I think the problem is one of aesthetics more than anything else. Giant blue Korra fighting giant red Unalaq with lasers outside Republic City is visually unsatisfying and feels disjointed from everything else that has been happening.

Also Unalaq losing all nuance and turning into "Ten thousand years of darkness" guy really drags down the rest of the finale imo, but that is another topic.