There is one thing in common with the PeterMJ (Spider-Man/Marvel) and Romione (Harry Potter) shippers; both of their pairings are 'canon' and have everything going for them thanks to favoritism by the author (J.K. Rowling)/ comic writers (Marvel). Despite all that, these
'canon' shippers aren't satisfied with having an already huge following and seek/continue to tear down on other ships (Harmione & PeterGwen) just to push their pairings to front and center. People like these are why we hate fandoms.
I can't comment on the Marvel portion of this ask as my knowledge of that universe is limited in scope and I'm not an active member of its fandom.
As for the Harry Potter part...
I think Romione fans have the issue that while their pairing is canon, it's not well developed in the text (very few pairings in Harry Potter are). We're told that they get together, marry, and have children, yet we don't actually see a lot of development of the relationship blocks that would support the longevity of their marriage. Yes, they went through the war together (although Ron abandoned them for a good portion of that) and yes they were friends, but their friendship was highly contentious and they spent significant portions of each year not speaking to one another or not hanging out (first year before they were friends, second year when she was petrified, most of third year, part of fourth where Hermione was more with Harry, part of sixth year, and part of seventh). Now, technically, first and second year don't count, but honestly, the only year they spend the entirety of together where they don't have a major fight is their fifth year. We know they like each other from Hermione's actions in book six and Ron's in seven, but we don't really see why they do, why that would turn into love rather than a teenage crush, or why they'd make it as adults.
Supposedly Hermione likes Ron because he's funny. Supposedly Hermione's maturity makes up for Ron's lack and he gets her to open up. While there's something to her liking someone who can lighten things up, if that's all then she could just as easily have liked one of the Weasley Twins instead. As a Hermione-type myself, I've often liked guys who were funny, but them being funny isn't enough. The guy I dated when I was 17 was funny, but he also lacked drive and dedication and in the end that made us incompatible in the long-run. Yeah, it's nice to have friends that can make you laugh, but in a partner for life you want someone you don't have to nag all the time when you need them to do something.
We also know that JKR admitted that Romione was her personal wish-fulfilment, clinging to her original idea of the plot, whatever that was, rather than a natural outcome of what she actually wrote. She also said they'd have needed counseling and their relationship was further questioned in the Cursed Child.
This puts Romione shippers on a weaker footing than most canon-pair shippers in other fandoms, making them defensive. Yes, they have the benefit of canon, but not the textual support, other than 'well, our people got married.' They can be threatened by Harmony supporters, who have fairly well crafted arguments grounded in the text for why Harry and Hermione make sense as a pairing. This makes Harmony a greater threat than pairings which are more loosely grounded in the books: Dramione, for instance, is never going to happen in a close-to-canon universe, but it takes very few tweaks to make Harmony work. Additionally, many of the arguments used in favor of Romione are equally as powerful for Harmony, such as the length of their friendship. Indeed, many of the arguments regarding Hermione's feelings for Harry specifically are a lot stronger, because we do see instances where she chooses Harry over Ron, such as in the seventh book.
It is unfortunate that these tend to become rather nasty fights that devolve into personal attacks on the sanity, demeanor, or reading comprehension of the Harmony shipper. I've seen many people go after another Harmony shipper on here pretty nastily, and I've heard tiktok and instagram are worse. Some screenshots I've seen are basically Romione shippers vowing to spread negativity about Harmony just because they don't like some of the fanfic people have written. As if it weren't possible to just, you know, not read the fanfic you don't like.
It would be nice not to get people randomly calling me a bitch in my inbox, but you know, such is life. I don't know if they dislike me more for shipping Harmony or Jily at this point but I really don't care.