The King: Eternal Monarch is the most complicated drama I've ever witnessed and even after reading bunch of threads and explanations about the episodes and parallel universes, I still can't wrap my head around this story. You have to be really observant, pay attention to every little detailed and even take notes just to grasp the direction of the plot. The different universes are the main pain in this drama. Especially at the end, it became really confusing and annoying, I've never known in which universe we are now, and the attempts to understand the plot, when it's pace was so fast, took away all the joy I had.
And to be honest, the beginning was really boring, the side plot about japanese threat unnecessary (probably they put it just to maintain the typical 16 ep duration), the whiplash of Tae eul emotions gave me headaches. In first eps she is so against Lee Gon - and as she should, she didn't know him then - she's really wary of him BUT by ep 3-4 they already kiss kiss, and then bum, the love confession from TE (which was the most unemotional and driest confession I've ever watched) and whooop! after 10-11 ep we have crying and love-confessing Tae Eul almost in every ep - it's just not really believable! (I think it's because soemone noted the lack of emotions from TE side and decided to change that... but in my opinion the change was too drastic at this point of drama).
And of course, the most annoying thing this drama had - THE FREAKING PRODUCT PLACEMENT. The could tattooed all the promoted logos on the Lee Minho forehead and it would be less irritating then puting products in almost every freaking scene (WE GET IT, YOUR BBQ CHICKEN IS GOOD, STOP MAKING LEE GON EATING ONLY CHICKEN, THE POOR BOY MUST BE REALLY HATE IT AT THIS POINT).
Of course, there were some ray of sunshine - for example the scene when Lee Gon comes Tae Eul to the rescue (CHEF KISS! ICONIC!) and... basically all the scenes with Jo Eun Sub (Woo Do Hwan was the best part of this all drama, his acting was superb, and at one point, I just watched it only for his charakters).

All in all, this drama wasn't the best Kim Eun Sook work, I would much rather put it at the The Heirs level than Goblin or Descendants (it's not even clode to this two drama jewels). In short: this drama is really complicated, but not really satisfying.