The two are of different genres and formats. From the start 19 days has the slice-of-life sorta setting where each chapter is a short snippet of the lives of 4 teenagers. There is an over-arching story, yes, but the readers come in expecting these comedy snippets.
Meanwhile Here U Are takes a completely different format. It's not a collection of shorts. It's a continuous love story. Readers come in expecting relationship development, and when they don't get what they expect after 87 chapters, they get frustrated.
It's kinda like comparing a comedy sitcom, The Office, to a romantic love drama. I don't know if you have watched The Office, but each episode kinda resolves on its own. There is an overarching theme (like Jim and Pam's love story), but viewers don't tune into an episode just because of their relationship. They tune in for the laughter. And when we get a small bits of romance, we all squeal out of excitement. But it is a whole different story for a romantic love drama. This is why sitcoms like The Office can have 9 seasons, but a romantic love drama only has like 20 eps. For a love drama, you tune in for the relationship, and if the whole episode just involves side characters' fillers, you are left disappointed.
Also given that each chapter only has 8 pages, Jianyi and Zhengxi still have more development than Lihuan and Yuyang, which says something.
Speaking from someone who enjoys both of them, the main difference between them is tone and timing. With 19 days although it's very long (despite the chapters being relatively short), it has a larger focus on comedy and we constantly get to see interactions between the main 2 relationships.
However, with Here u Are, it's a bit more serious (even if it does have lots of comedic elements), and the author, especially with the latest few chapters has a tendency to focus on relationships at times where there's an ongoing angst or hurdle between the main couple. With Here U Are they have an abundance of relationships, and some people just want the main couple to be happy and set before working on 3+ more ships. Like, I enjoy all the other ships in it, but the timing when they choose to focus on them may frustrate them. At the end of the day, people want the problems and angst to be solved, but neither rushed nor overextended. Currently, a lot of the fans want the current problem to be overcome, however they are getting a side ship being worked on instead, which means the rate at which the issue will be solved will be extended further. For some, this makes it more difficult to care for these other relationships, because they're still worried about the main couple.
Also, Here U Are (HuA) feels more like 2 steps forward, 1 step back; because even when they overcome an obstacle something inevitably hinders their relationship. 19 days is a more progress based, where they are constantly putting the couples together where they will grow and grow. 19 days also doesn't really rely on other characters getting in the way, which is a trope some don't like - like there is no third love interest or hindering feelings for someone else that takes up a significant amount of timing, and even if there are misunderstandings they often feel less angsty.
Just my 2 cents tho
I agree with everything the two above have mentioned. There's one thing I wanted to add regarding tone mentioned earlier. 19 Days overall has a comedic/silly atmosphere with a few sad/angsty chapters in between. The angsty problems are resolved in a satisfactory and quick manner so the reader isn't left with a bad aftertaste for long, because of how quickly problems are addressed. We are also "compensated" with cute, light-hearted romantic scenes following the drama. On the other hand, HUA is continuously angsty without much compensation. Once problems are solved, they only reappear in different forms over and over (ex: insecurity about homosexuality). Hate towards HUA is not an issue of pacing since both stories are incredibly slow, the problem is the built-up frustration of having to witness the same angsty situations reappear without a conclusive solution. I hope that adds more nuance to your answer!
I'm not trying to pit two completely different manhuas here but in terms of an immensely slow burn, '19 days' and 'Here U Are' can be compared I think? So here's my question, why are people so much more frustrated with 'Here U Are' than '19 Days'?
'19 days' seems to be much slower in my opinion but I don't feel angry with it? Is it because it didn't promise us anything at all from the very beginning, that's why we're more patient with it? While HUA went with a more linear approach then went off-track? What am I missing here?
I hope I'm not being rude to anyone by asking this, I'm just genuinely curious as to why more people are much more frustated with HUA than 19D. Hopefully you guys can answer haha ヾ(❀╹◡╹)ノ~ Love both manhuas regardless. Cheers!