They just hate harem because they like cheap drama and the "will they? won't they?". They treat "harem" romance more like a competition to see which ship is canon rather than considering the actual romance writing. And to be honest, most pseudo-harem romance sucks, because they just throw away chapters of development with no proper resolution because the author can't afford to piss off readers because they get too attached to their ship. And if they do end in monogamy, it's in a way that generates the most mixed reception as possible because they're too wuss to write a proper conclusion that gives justice to every character involved. IDGAF if it doesn't end in a harem, at least just not end it on one chapter and screw everyone else. I've only read one pseudo romance where the MC actually makes it clear that he's only interested in the main girl and rejects everyone else along the story, and it was good because the author knows what she's doing and give proper resolution to multiple routes and relationships. The rest can go to shit for all I care.
The entire "harem" thing in romance mangas is really just the author trying to create a "ship" war which made the quality of romance mangas go to shit because the author would focus more on milking the audience rather than to tell an actual romance story. People who read romance for the ships don't really care much about the actual romance plot, since most of them would make excuses when a love interest does something unhinged / awful for the sake of love or find any material that they can use to legitimize their ship (mostly for antagonizing other readers). Trust me, I've been reading the comments on a bunch of "harem" romance and it doesn't really end well.
There's nothing even wrong with true-harem endings as long as it is properly developed. And it's not really bad writing just because it satisfies every reader except the ones looking for drama and ammunition for their ships. Webtoon Character Na Kang Lim ends in a satisfying harem and it's considered as one of the best romance comics of all time. It's all comes down to good writing and romantic development. IDGAF if it's not "realistic" because romance is "wish-fulfillment" at its core. As long as it is well written, I don't care on how you end the romantic teasing, at least make it feel earned and not a cop-out or divisive.
Mangas that end in harems are way better written than the ones that don't because the author gives a shit on fleshing out every single love interest and make the harem feel earned rather than given. And honestly, true-harem romances tend to have the best waifus and have the most well-written MCs in general. There's just something about pseudo-harem romance characters that just pisses me off because everyone involved has to be roped into cheap and forced drama for no reason.
"What about the bittersweet feeling?" Yeah, you can do that and not make it suck ass. If you somehow can't write multiple LIs in a romance plot where the MC will reject them in a way that gives them proper closure to pursue the one he/she loves, then you deserve the backlash. Na Kang Lim is able to do it with her ex, I don't see why this isn't possible.