In my personal opinion, the latter half of the show fell victim to haphazard pacing and overuse of deux ex machinas. There were too much "gotcha!" moments to the point the tension lost its purpose because the viewer was reaffirmed that things will turn out well no matter what. In a fluffy romcom this kind of reassurance is exactly what you need, but I felt like it didn't fit right with the initial vibe of *this* show.
Also it had little bit too many characters (at least 2 concubine characters could've easily been cut off) and as much as I love our Queen, I felt like her demeanour was over-dramatized so the credibility of her character leaned a lil bit much to the wrong side. And the villains were quite one-dimensional.
BUT

From beginning to the end, Under the Queen's Umbrella had its heart at the right place.
It did a number of things a lot of sageuks failed at doing for literally ages after the end of the proper long sageuk era about 10 years ago. Such as;
1. Not portraying the palace women as helpless flowers waiting for the king in their rooms all day long. They had STUFF to do. They were capable and intelligent individuals of their own right. They were constricted in lots of ways than not, yes. But it did not mean they always kept their heads down and laid low.
2. Making the harem ladies care about things other than being jealous with each other all the time. Women aren't so narrow minded, yknow? In a society where polygamy was the norm, I'd believe a majority of them had other priorities than hating on another woman who is no less of a victim of circumstances than they are.
3. Not pretending like the King was monogamous with one true love even though the guy literally had a harem, AND not pretending like he'd be an evil person otherwise.
The King of Under the Queen's Umbrella was refreshing in the sense he had a harem, yes. But also he was a nice and fair guy. He and the Queen had deep mutual understanding and respect towards each other. I do wish that the drama showed more scenes of him with other concubines as well tho.
4. Not repeating the trope of overbearing mistrustful Royal Father and the Crown Prince who is always under too much pressure + other Princes feeling neglected. This King (and also the Queen) cared for their children first and foremost, and would listen and give them benefit of the doubt before blindly believing what outsiders have to say. This drama had some of the most wholesome parent-child scenes in Sageuk history in my opinion. Regardless the child was a legitimate Prince or not, their treatment was all the same.

So yeah, if you think you'd love seeing the above 4 points in a Sageuk, Under the Queen's Umbrella is a must watch in many ways than not. Do give it a chance!