I don't know how unpopular those are, it probably depends on which circles you apply them to but here are some I thought of:
1. I personally don't like survival shows, when it comes to watching them.
After a survival show is over, I like researching about it and the members and see groups they are in/will be in, but during the airtime of the show I just can't stand watching it. I don't wanna get invested into some that will not make it, plus these shows are kinda rigged a lot of the time (well, especially mnet shows) so I just don't wanna see that. All I see happening from survival shows are unnecessary fanwars (even before a group is even made! Like fanwars make no sense even when there are groups but starting so soon? Some people are just in it to be haters.).
2. I don't get why a lot of people are so invested in the idols private private lives. The fact dating scandals are called scandals to begin with is so wild and that people are like, ranking couples or something is wild to me. You have no business with that and you shouldn't care about it... Same with fans being actually sad about an idol dating someone, shows how much you don't even care about the idols as a person. I mean, sure, those fans are not the majority and are just the loudest, but just had to give my thoughts on that.
3. SCREAMING at concerts. I've mainly been to metal concerts in my life so this switch to kpop might sound unfitting to begin with but, whenever I see recordings of kpop performances, I hear everyone high-pitch screaming their lungs out as if they're being chased by a murderer or something... Fan chants are nice, but that's not what I'm talking about. I can barely even hear the singing?? I don't think I could go on a kpop concert like that but maybe there are some groups where it isn't as bad, although I doubt that lmao... Nothing against the people doing it, you do you, but I really don't see the point of screaming so your throat hurts and everyone around you gets their ears hurt.
4. This one is definitely an unpopular opinion, but I absolutely haaate cheesy romance-y/fan-service songs!! It took the longest time for me to just try and ignore the lyrics for so many songs and just enjoy the music for the sound and the voices, and if there is one, the choreography. Especially when it's uhh first-person?? singing?? Like the song sounds like it is directed at you, the listener. I just cringe so hard... But a lot of my favourite songs sound-wise have those kinda lyrics so I just got used to it more and more. Because of that I appreciate it even more when a group has an interesting/dark story in their concept or music video going. But at the same time I absolutely adore those over the top cute concepts, so like, uhhh???
Which brings me to...
5. I have a weird hate-love relationship with aegyo. Like, I find it adorable at times when idols do it or are just naturally cute, but sometimes, especially when it's "forced", as in they're being told to do it, I just physically cringe. Even worse when they do some fanservice romantic acting bits towards the viewers, most of the time it makes even the idols cringe because it is so weird. But I can't complain about all of this existing, because at the end it is what brings a lot of fans so fair enough. One might argue that this is one of the things enforcing the para-social relationship obsessed side of the stans, and to an extend I would say yes but at the same time, those exist for every fandom, regardless of people doing that or not, so I don't really think there's much harm in it.