Let me start with a disclaimer that I kind of gave up on this show (more than once) but still wanted to write a somewhat robust review so I forced myself to skim the last two episodes. This means I might have missed some important nuance (surely) or events somewhere in the show and my review isn't super detailed.
And yeah I skipped the side couple's scenes. Sorry! I heard they were good but I just didn't care to watch.

- Plot and Writing -
The writing for Hidden Agenda is pretty bad. I don't think it's awful and unwatchable (on writing alone) but the show felt directionless, as if it was being written in free-form. This could just be a taste thing. I'm not a slice of life person, which is a source of potential bias against Hidden Agenda and it's very simple story. But I don't know. Either I'm wrong, or the show is bad.
I think perhaps what bothered me most is that the show is called "Hidden Agenda", the same agenda by which the show is driven forward. When this ordeal is resolved before the climax of the show (instead of WITHIN the climax of the show) it induces audience whiplash. I imagine it's how people feel when they talk about Riverdale's billionth season being about completely random shit. Except GMMTV shows average 12 episodes. Which means the hidden agenda could have been drawn out to fill those 12 episodes. Instead, the story wastes a lot of space and leaves a lot to be desired. And I saw many MDL commenters sharing this same sentiment.
However, I've seen people praising the simplicity and feel-good aspect of Hidden Agenda. It's an "easy watch" (not for me, which I'll talk about in the acting section) and doesn't raise your blood pressure or expose you to intense content. Hidden Agenda is quite comparable to 2gether the series in this way. The acting is also quite comparable. I dislike both shows.

- The Hidden Agenda Reveal (ep. 9) -
There were a lot of mixed responses on MDL to the Hidden Agenda reveal (and all of Joke's actions leading up to this reveal). Personally I was on the side of finding Joke's approach manipulative in the earlier episodes, but the framing of the show wasn't clear enough to show this to the audience. From what I read about the manga from the comments, it is more obvious that Joke is manipulating the situations he and Zo are in as a means of getting closer to him. "Manipulation" not being an inherently sinister word, but I personally found Joke's use of manipulation too heavy-handed. I was on Zo's side here.
When it was revealed to Zo that Joke had this "hidden agenda" the whole time, Zo becomes upset and expects an apology. For this, half the MDL comments seemed to believe Zo acted hypocritically. "Didn't he do the same thing?"
My thing is, Zo's hidden agenda with Nita was acts of kindness, like trying to carry her stuff, or inviting her to the library under the guise of studying when he actually wanted to read her a story and gauge her response, asking someone she knows for advice on how to date a person, aka actually getting to know her. I think that's pretty harmless. Meanwhile, Joke's approach was being a weirdo. Backing people into walls, sneaky wedding rituals, "we have to go to my house oh its coincidentally a huge mess omg its midnight already I guess you have to sleep over promise I won't do anything weird! Change your style, change your food preferences, I'm super possessive around your friends." Like how is that the same as what Zo did?
I was also stunned by the fact that Zo is one of the least perceptive characters of all time, allowing Joke's hidden agenda to work in the first place. If Zo was a bit less dense he would have realized Joke's hidden agenda sooner. But I won't judge. He has trouble reading people, he has mommy issues, etc. He's a troubled boy. It's not his fault. But what bothered me about the big reveal is that, at that fucking point, the two of you are already dating, you know that he likes you, so did you not put 2 and 2 together? While I don't blame Zo as a character, I blame the writers who gave him so much goldfish-brain that it also deprived the audience of retrospective thinking.

- Supporting Characters -
One noteworthy scene was the Wave and Trin flashback story in episode 10. It was one of the few times in Hidden Agenda where I actually stopped and watched the full scene without pausing or skipping ahead. This is probably attributed to the relative acting talent of these supporting actors, as well as the way it stood out from other scenes.
The problem is that this scene served as insignificant exposition within the greater context of the show. It was a singular, self-contained conflict that explained another singular, self-contained conflict. This wouldn't have been an issue if the screenwriters had found a way to incorporate other self-contained conflicts throughout the show. Sprinkling in the supporting cast throughout the show, instead of just oh there's a stalker at the beginning, an old crush in the middle, and some random beef between seniors towards the end. Maybe we could have learned more about Zo's friends, maybe we could have learned more about Nita (because she was pretty cool), maybe we could have seen a little more of Joke's grandma (she was the best).

- Joong and Dunk and Acting -
So there's the enormous elephant in the room surrounding acting talent. Specifically the two lead actors, the stars of the show, the people who the entire show revolves around.
Personally, I find Joong and Dunk so hard to watch. Both together and separately. I have to do a lot of pausing and skipping, a lot of trying to hold myself together, because I'm very susceptible to secondhand embarrassment. I may have even had an easier time watching Star In my Mind, since I didn't know the actors yet and it was their first time acting together and you can only go up from there, right? Well they haven't gone much farther up yet. Honestly there was a moment in ep 10 where Dunk delivers a mini "you're being an asshole" monologue in the scene between Zo and Wave. That was pretty good! For Dunk standards, that was pretty good. But overall, the acting in Hidden Agenda was just not good enough.

But I want to say something very extremely important. Whenever you go into the Hidden Agenda comment section you're going to see 10,000 comments calling Dunk talentless. This may be true. But you'll also see those fans who try to desperately save Joong from from getting joint criticism. Saying Dunk is dragging Joong down and that Joong deserves a better partner and his career is being impacted by the evil dark energy that is Dunk. This is delusional. You just think Joong is sexy.
Dunk may objectively be the worst actor in this cast, and you might even expand this debate into the greater GMMTV roster, but that doesn't mean that someone who is better at acting than Dunk is a good actor. Joong may not have won 1st place in the GMMTV Worst Actors Contest, but he's still somewhere on the podium.
Joong is, once again, playing the monotonous quiet type. He's been playing discreet characters, in contrast to Dunk who's been playing the clumsier, nerdier, more animated and outwardly emotional characters.
This is tangential, but I want to note that this dynamic commonly follows couples in BL, and it's typically the case that this dynamic is superimposed onto the ever-present pest that is the top/bottom dynamic. Whenever the trope is reversed, it becomes a subversion. I find it the case that this dynamic follows masculine/feminine expectations, which are then projected outwardly onto the actors. Not only is a lack of acting talent more evident when your character requires more expression and dialogue, but the trend seems to be that, unless this actor is extremely capable with a solid reputation, it'll be the "bottom" who receives criticism disproportional to their performance.
This might be a hard sell when I'm using JoongDunk as an example, considering the fact that Dunk is truly a terrible actor. The criticism he gets is valid. But given the characters JD have been typecast as twice, and the comments that Dunk gets compared to Joong, I think it's fair to bring this issue up here. Because Joong is also, truly, a terrible actor.

I do think these two have been kinda victimized by GMMTV. This is their second project together and both shows were shit. That sucks, even considering acting talent. I think with good screenwriting and/or directing, even bad acting can be concealed within a show. Midnight Museum managed to do this and that show wasn't even good. Of course with GMMTV, and especially with JoongDunk, quality isn't the goal. These two actors are young and attractive and they're fanservice menaces. The only thing being monitored right now is profit.

But there is always room for improvement! I believe deep down in my heart of hearts that Joong and Dunk can improve as actors and put on a decent performance. I hope they get cast in a show with good writing and directing and gorgeous cinematography and good hairstyling and makeup. Joong and Dunk are too good looking to waste. And they might be terrible actors but when it comes to kissing they really go for it. That's kind of impressive. I've never seen two BL actors actually allow their bare skin to touch in a scene before. JD have courage and it should be sent to the acting workshops and used by good writers and directors.

- Overall -
Hidden Agenda wasn't good. It might have been watchable with different actors, or with the same actors and better writing. If it had different actors and better writing, it might be an excellent show. But those are hypotheticals and Hidden Agenda did not have different actors or better writing. Some people enjoyed it. I'm happy for them. I did not enjoy it and I would not recommend it, even as something to have playing in the background. If I was studying or cleaning my room and I heard a line delivery from Dunk or looked up to see Joong furrowing his eyebrows on my screen I don't think I'd be able to handle it. You're stronger than me if you can. But the intro OST was catchy I might go listen to it after this.