First of all, I don't believe the ending from a character perspective. At this point in the story, Jingnu and Qi Yan have gone through a lot a lot. Frankly, I would argue a lot of what happened beforehand was worse than what kicked off the ending. I simply do not really believe that it makes sense for Qi Yan to run away to that extent especially when Jingnu gives both strong reasoning for why she's doing what she is from a political perspective (which, as a reminder, is essentially for survival considering how they're still in the relatively tumultuous and politically unstable period after a new emperor, a female one at that, rises) and gives a plan for how they can be punished after the political situation calms down. That's a lot more than can be said for a lot of the earlier conflicts.

I understand that the idea is that now that she's accomplished her revenge and Jingnu is in a position of power, Qi Yan is a lot freer to leave. However, clearly she doesn't consider herself as having fully accomplished her revenge, otherwise this conflict wouldn't exist in the first place. Add in how much the previous conflicts have left both of them reflecting on how much pain and suffering it took to finally be together, and I just don't believe Qi Yan's actions as a response.

Second, I dislike the ending from a morality/fairness perspective. This ending would make significantly more sense and even be in some specific ways cathartic if the story largely involved Qi Yan constantly making sacrifices for Jingnu. However, I would generally argue that over the course of the story, both of them made reasonably equal sacrifices and compromises for the other, and if anything I would argue that Jingnu compromised more for Qi Yan overall. (Feel free to take or dismiss this point as you please; I ended up with the feeling that Jingnu did more for Qi Yan over the whole duration of the book, but trying to justify it further would require the aforementioned reread that's not going to happen so if you feel differently that's fine.)

The counterargument that I can see involves the fact that Qi Yan lost more (her entire tribe and way of life) than Jingnu, but I disagree for two reasons. The first is that I would argue this isn't really relevant to the relationship directly between Qi Yan and Jingnu - if anything, Qi Yan reflects at several points that she's hurting Jingnu despite how Jingnu wasn't really responsible. The second, closely related, point is that I don't believe Qi Yan would hold that against Jingnu. Again, Qi Yan's internal monologue considers herself as hurting Jingnu for things that are not Jingnu's fault, and it was never really brought up even during earlier conflicts where Jingnu compromises because she feels that Qi Yan deserves vengeance. It's possible that Jingnu becoming the emperor makes things feel differently to Qi Yan, but frankly if this is the case I'm very unconvinced it's backed up enough by the actual text and starts to feel like making excuses for the plot instead of examining it.

Finally, I think this ending fails to provide a satisfying conclusion. If the goal was to write a tragedy, then write a tragedy. This ending is claimed to be a happy ending, but then even ignoring everything else, it fails from both a background and foreground plot view. In terms of background plot, the idea that Jingnu can just stop being the emperor and that Qi Yan finds some magical fake death pill does not feel adequately backed up.

In terms of foreground plot, there's quite a few issues. After a big melodramatic plot where both characters reflect multiple times that "it sure is nice that after all of this struggle and suffering we've finally managed to get together, this love really is worth holding onto", ideally a happy ending would provide moments of actual happiness for the characters; it's a reward, like how when a book writes a big political struggle it'd be pretty disappointing if the aftermath was relegated to one sentence at the end of a book, or if a big mystery ends with the criminal just getting away anyways or getting arrested offscreen. There's a reason Sherlock Holmes lays out all the little clues instead of just going "anyways I figured it out, dude's arrested, peace".

Another issue is that amnesia as an ending is almost always an awful idea. Introducing amnesia that isn't later reverted into a relationship naturally makes the relationship feel way more hollow. It strips out all of the complexities, the growing of feelings, the struggles and learning that makes a relationship, especially one as tumultuous and complicated as this one, special. The fact that it's implied "oh well maybe she'll get her memory back later" just makes it worse; at that point just write in her getting her memory back then.

Ultimately the ending feels extremely rushed, has very questionable setup, and even if you don't care about any of that, it more or less feels like it puts the actual happy ending x years down the line, and we just never actually get to see them talking over their problems, reuniting for real, living a life they enjoy, etc.

the romance between Nangong Shunu and Xiao-Die. I get that the author wants their romance to be between two women with a traumatizing past with men, but I felt rather uncomfortable over the fact that Nangong Shunu fell in love with Xiao-Die's innocence and infantile purity, engaged in a s*xual relationship with her while she was still mentally incapacitated, and yet it was never brought up either by the characters or the narrative.

See I'm pretty blasé with romance tropes in fiction. Explorations of unhealthy, imbalanced, or non-consensual relationships is fine and safe in fiction and I really like them when it's done well. It doesn't have to be put under irl moral scrutiny all the time. With that said, maybe I do have a bottom line, and it's when one half of the pair is mentally in a child-like state. Xiao-Die's condition is temporary and she does recover her adult mentality later on, but it bugs me that it was never a point of conflict for the characters or the story, and that the main conflict of this pairing was only that Nangong Shunu was the daughter of the enemy. I think I would've been fine with it had Xiao-Die been given time to develop outside of Nangong Shunu's protection after her recovery, but even the stabbing incident was a total accident, and was even more somewhat a calculated move on Nangong Shunu's side. I would say that the Shunu-Die romance somehow stunted character growth/potential for both of them.

between Qi Yan finding her sister up until Nangong Jingnu's ascension,

the revenge IS fulfilled, leaving Jingnu alive!

Why was Qi Yan said to be the prince of the grasslands and have the wolf tattoo of a male descendant? I can understand hiding as a man for revenge since it will give her more influence but why was she raised as one? If I didn't misunderstand the author said that she even had a younger brother, so I'm not sure why she had to hide.