Both have star-crossed lovers as the main couple with a one-sided love triangle between the ML who works in/ends up working in the palace, the high-born but unpowerful FL, and a somewhat foolish and childlike emperor with a violent temper who was childhood friends with the FL and is now obsessed with her.
Both dramas depict a forbidden romance between the man serving the monarch and a woman who is a love interest of this monarch (king or emperor).
Both dramas have a fair amount of comedy and a flowerboy ML.
The genre and the vibe the two dramas give off are very similar. You have romance as the main plot with a political revenge plot going on in the background. The two FL's are very similar with their 'crackhead' energy. The ML's are very different though, which results in very different dynamics between the leads. Mr Queen had an enemies-to-lovers theme, while Unchained Love has saviour to lovers? All in all, I believe if you enjoyed one, you'll enjoy the other.
Both are historical dramas in which the MLs are pretending to be eunuchs to get revenge. The FLs have similar personalities as well.
In both dramas, the main male lead (ML) is the star of an investigation and enforcement force with many powerful political players seeking to manipulate or neutralize him. He's sucked into the middle, until the main female lead (FL) inspires him to find a way out
In both, the FL has her own ambitions and desires for her life that the ML is happy to support and pursue. The FLs take their fates into their own hands and come up with their own strategies.
A Dream of Splendor however is better produced and is less cruel and sadistic. Also a little less romantic and a bit more focused on female friendship than a main romance.