Very bad. I find it mindboggling that there are so many 5 star reviews. I think another reviewer here pointed out that they are fake. I would hazard a guess that the real rating would be somewhere around 20-40% approval/"liked."
ANYWAY. The story is just straight up bad/boring/poorly written/WAY TOO unbelievable, even for a drama. Characters are so badly written that I can't bring myself to empathize with any of the good guys or be mad at the bad guys.
The crown prince's uncle is so over the top villainous with no real reasons whatsoever for his bad behavior, he's like bad for the sake of being bad and to contrast with the crown prince's excessive self-righteousness/"heroism." Same goes for the Korean-born Ming diplomat. He's just a man-child (there's no way he would have become "head eunuch" with his awful behavior and background as a Joseon foreigner) that's straight up violent. He supposedly has a reason but honestly, it's so terrible and beyond unrealistic.
Rowoon's acting is good, Park Eun-Bin does as well as she can as a woman passing for a man.
The show is filled with K drama cliches and K drama elements, but it is not in any way interesting at all. As others have said, super waste of time. If you don't mind a slightly older show with lead actress playing dual roles, watch "Oh My Ghostess" instead.