This is after all, my own take after watching the drama.

Basically the first part of this drama is excellent.
The actor are amazing and the action is well mixed with other moments so you never get bored.

Now the second part, I dislike how they throw away in 5 minutes at least 2 plot points that were really interesting.

But Ok, maybe they needed more episodes to solve the final plot point which is, after refusing to become the royal concubine for ages, why did Duk Im say yes?

The second part is not as good, as it turns grimmer and grimmer. But it presents a very modern idea that I did not expect to find in a historical drama: power imbalance will never lead to love.
Duk Im has to let go of who she is, and what she believes, and therefore she becomes unhappy. So for almost all of the last part, Duk Im as a character is a melancholic, except for a few happy moments, queen.

The drama takes a really brave moment deciding not to give them a happy ever after, but a "I loved you, but I should have loved myself more". The writers didn't let Duk Im relent, she, until the end, was clear. She may have loved San, but she wasn't happy.

And then we arrive to one thing I really didn't like, what I want to call "The producers ending". The ending for me kind of contradicts the whole tone the drama took.
I would have loved it more if at the end we are transported to the first time they both meet in the palace grounds or somewhere else, but when San is about to call her, he remembers her last words, and she lets her walk away. I think this would have shown a better character evolution of San that the ending of this drama.