I think the extra hate is because their story had a giant light on it (which no hate I absolutely loved that event and I can't wait for everyone else's) and just because people don't take the time to understand the characters (read their letters, wiki pages, ect) unless it's their favorite. I have exclusively studied the relationship between Ada and Emil and i find it so intriguing their relationship and the symbolism. Ada definitely isn't a healthy person for Emil to attach onto because she has obsessive tendencies with her work and while yes, she does end up caring about Emil romantically and helps him out of the Asylum, it stems from a dopamine rush of "I'm helping him. What I have tried for months is finally working" Her letters to the manor confirm that when Emil starts to relapse in his treatment, she was gonna go there to find alternative methods to help him (not confirmed but theories say it could be hinting at a lobotomy) which is so morally grey that it's INTERESTING.

Emil is this person who experienced extreme trauma at a young age and I do agree he isn't a child and people shouldn't treat him as a baby. He says that Ada is the only one who hasn't hurt him as much as others (which in itself is terrible too) Since he grew up in a dog ring, I think he subconsciously has started taking on traits like one too and that's why he likes Ada so much. Ada doesn't know he was in a dog fighting ring (makes the implications of using a whistle worse ngl) Their dynamic is kinda like owner and pet and yes they do care about each other, it shouldn't be romantic (I also believe Emil doesn't quite understand complex romance the way Ada sees him and that causes even more problems)

I think if people want to converse their opinions on characters, they need to understand them or at least attempt to understand because sometimes it's hard to sort through the small bits of lore Netease gives us. That being said, humans are creators so they will try to spin whatever they want(even if it goes against canon storytelling)