This drama definitely had its problems but I think many of the reviews are unnecessarily harsh. The show does start off slow with a lot of build up of the characters as "perfect". I think a lot of these earlier episodes would have been better used setting up the plot and developing other characters but thats off topic as that is not what happens. Instead the characters are mainly kept 1 dimensional and there are a lot of little hints and sub plots that the writer tried to embed into the story (the writer on this obviously thought they were being more clever than they really were).

There is a lot of hate online that the leads had no romance and that their acting was bad. Personally I like LMH and think he did a good job with this drama and pulling off the arrogant King. I think their were a lot of unnecessary side characters who them we really dont get to learn about because the writer waste all of our time in the first 10 episodes. On top of that the timeline is confusing (episode 12 I literally was like "WTF that doesnt make sense").

This drama is not all bad and does have a cool and interesting premise, including sci-fi, multi dimensional, time travel, and I personally love the "kingdom of corea."

It just fails to deliver on all that it promises.
I think it might be worth rewatching just to see if it make more sense the 2nd time around (I kinda doubt that it does).
I also think that if you are in the early episodes and having issues I suggest just jumping to episode 12+ as most of the story seems to really take place in episodes 12-16.

I still give it a 7 overall because I actually enjoyed the main characters and the concept of the drama even if I dont think it was executed well.