Reunited Worlds claims to be part romance, part drama and part fantasy. Except it's really not fantasy and tends to be a whole lot of confusing.
The show follows a fairly fantasy premise. The once-dead Hae Seong returns to life as if he never passed away, while finding out that everyone he knew has moved on in the 12 years since his death. There's a mystery thrown in that Hae Seong works to solve while also adjusting to the new world and life he lives in.
That being said, we never really explore the fantasy aspect of the show. Hae Seong comes back, some (but no where near enough) questions are asked and then...nothing. The focus shifts towards the mystery and the relationship between Hae Seong and Jeong Won. Which isn't necessarily bad in itself, but leaves a huge gaping plot hole with regards to anything that has to do with Hae Seong's return. And considering it's such a crucial factor, it really needed more addressing.
The show's strengths is the acting, with Jin Gu being as great as ever while the rest of the cast is serviceable.
The weakness is the writing. Along with the plot holes, oftentimes the plot seems like it was written or guided by a child. Very little of it seems true to life and many of the characters actions feel like they're being viewed through the lens of a child; you want something done, it'll magically happen within the next 24 hours, personalities flip quickly, there's little questions about things that should be asked and areas of strong story-telling exploration get briefly skimmed over. That's not to say there aren't any highlights; the siblings and friendships are great, and Jae Hyeon is a unique leading man. He loves the girl, he's suave, smooth, rich but he's also dorky, messes up hilariously and is actually willing to give the female lead autonomy which is amazing.
Overall a recommend only for really dry spells or fans of Yeo Jin Gu, and really only with the fast forward button.