I think objectively, if this wasn't based on a manhwa, it could be called a really good show. Acting's good, cinematography is pretty. If the premise wasn't Hee Su, his journey, his relationship, but the intertwined relationships of these four high school kids, it would have been fine. It's clear that's what they wanted to go for and it does have some basis in the manhwa. By which I mean, the whole plotline of Seung Won getting closer to Hee Su by lying about his crush on Ji Yu.
However, it IS based on a manhwa, and a beloved one at that. And the source material, while it has all these side characters and supporting characters, they act like satellites for the main characters: the MC and the ML. They're there to support Hee Su and his relationship and they don't overstay their welcome.
I almost feel like they tried to do this with the relationship between Chan Yeong and Ji Yu, the sisters and their love shenanigans, by having them be parallels to Hee Su's relationship/s. But they ended up sinking so much time and backstory, at least for Chan Yeong and Ji Yu, that there's not enough left for the main couple.
It also doesn't help that they followed the first season of the manhwa (if I remember correctly) which does end with Hee Su and Seung Won getting together. Second season features another gay couple more prominently, but also shows you the couple life between HS and SW. But clearly, we won't get to see that. No matter what, that'll leave some folks (myself included) unsatisfied with where the show stops.
I know I read somewhere on Reddit that they had issues even producing the show, because of the themes, had problems with getting distribution for it. Maybe that's the reason for the watered down version we got.
All in all, my opinion is that the addition of the second couple together with their backstories brought little to the plot and wasn't that interesting. But did use up time that could have been spent developing HS and SW's relationship. Chan Yeong..I don't even know what his backstory is, ngl. Tennis. And something about an overbearing dad, I guess. As for Ji Yu, she's nice and she's a popstar on youtube. Ok, cool. Neither are as interesting or compelling as Hee Su though. Plus, I feel like we've delved into Chan Yeong tennis matches more than we have into Seung Won's backstory. Sure, he's alone at home a lot and has two moms and..erm...what else.
LE: There IS backstory for Chan Yeong in the manhwa, I forgot. And it's much more compelling and relevant than tennis lol.