I will start with a simple example, early on in the show there are many instances where Kubo-san invites the MC to do something but he refuses because he is worried she might use the opportunity to tease him instead. Hello? Am I watching the wrong show here? Was there a season 0 I wasn't aware of? Kubo-san has never teased you before, aren't you mixing her up with Takagi-san?
You might say no he's just on guard because he's a socially awkward loser, but that doesn't work either because the show established that the MC is not just some loser, he's literally the invisible man, he's never been bullied because he's never been seen before. There is no reason he would ever suspect Kubo-san is about to tease him except because the author read Takagi-san. And no it's not just some reference joke, because it happens multiple times, when clearly the MC needed some excuse to decline so it wouldn't be too easy for Kubo-san.
We are then introduced to Kubo-san's family members, which consists of her older sister and younger cousin, why cousin and not younger sister, even though she supposedly resembles Kubo-san very much? Because otherwise it would be too obvious that this is just an alternate version of Belldandy, Urd and Skuld from Ah My Goddess. The family dynamic is exactly the same, the sister is here to push Kubo-san and the MC together while the younger sister is jealous of Kubo-san's affection so she's here to keep an eye on their relationship not getting too close.
I'm not here to play plagiarism police, if the show borrows elements from other shows for a good reason I have no problem but I think this show just doesn't have anything that's both good and original. The "romance" story really hits the ground with a burnout, the main couple exchanging PINE accounts in the FIRST episode, Kubo asking Shiraishi out on a date, followed by a lightning visit to Kubo's home, then they exchange Christmas presents, Valentine's day chocolates, indirect kiss, porn magazines, all just in the first FOUR episodes!
The speed of progression is so fast even Kubo-san is confused, she doesn't realize she has a love interest when questioned by her friends even though she just gifted Shiraishi hand made chocolate in the shape of a giant heart! No she wasn't just lying to her friends because we see her inner monologue, she literally does not know, right after hand making her crush a chocolate for valentine's day for the first time!
Most romancon need some kind of gimmick to stand out, Takagi-san the tease master, Kaguya-sama will not confess first, I tasted my mysterious girlfriend's drool etc. The gimmick here is the MC being so socially dysfunctional that he is literally invisible, except he sometimes is and sometimes is not, the gimmick is lying to you because the author isn't creative enough to make it work consistently.
Sometimes the show claims that the MC is literally invisible to the point people will bump into him if he doesn't doge them, that he can enter and leave class at any time without anyone noticing, that he can just leave a convenience store without paying, if only the automatic doors would actually open for him. But that's just not always the case, side character still see him sometimes when it is convenient for the dialog. You might argue ok you're taking this too seriously, this is just a metaphor for how nobody cares about the MC except for Kubo-san, he's not actually literally invisible. But that doesn't work either because Kubo's older and younger sisters both see him immediately the first time they met, why? How? Are they also in love with the MC? Throughout the show this gimmick just doesn't pay off in any way, it's not funny, it doesn't create any unique plot points, it's just a thing that comes up once in a while for bad attempts at humor.
There's very important thing I think every romancon needs but isn't present in this one, while the MC can receive affection by chance or out of nowhere , it is crucial they must then EARN IT. This means they have to reciprocate this love through self-improvement such that they become truly worthy love interest. Shiraishi just doesn't do anything other than being a "nice guy" which is just lame, even by the end of the show there's no evidence that Kubo-san is a special someone for him. It just feels like Shiraishi doesn't care and Kubo is in this entirely one sided crush, which feels more like a tragedy than romance.
The show ends with a flashback on how Kubo-san noticed Shiraishi in the first place, she saw him in the yearbook, that's the flashback, that's the story. Really? Is this what you want to put at the very end of the season? Who cares? It doesn't change anything about the story or relationship!
I will end with a chart I've been working on, which I think better communicates what I think are bad about this show.
*Chuunibyou is based on only season 1, since it sucked in season 2.