I think it is less about hating the band and more about hating some of the overzealous "fans" that will hijack any conversation about metal and inject some garbage about Opeth being better than whatever band happens to be the subject of the conversation.
I haven't seen it much here, but it is similar to how Radiohead fans will hijack any conversation about rock/pop music with the same crap.
There is nothing wrong with Opeth, or most of their fans, but there is definitely something wrong with a small minority of Opeth fans that makes many people hate the band and the rest of the sane Opeth fans just on principle.
The conversation normally goes something like this:
person 1: Hey man, have you heard the new Immortal album?
person 2: Hell yeah, I loved it. It's probably my favourite album of the year.
person 1: Yeah, I love it too.
Opeth fan: Excuse me, I heard you talking about the new Immortal album. It's ok, but Opeth are better.
person 2: Dude, we're not talking about Opeth...
Opeth fan: Yeah, but Opeth are better
person 1: They are not even in the same sub-genre man
Opeth fan: Yeah, but if Opeth were in the same sub-genre, they would make Immortal their bitch. They are such better musicians and Mikael Akerfeldt is a much better songwriter
person 1 and person 2: Look mate, fuck off, Opeth are shit, we hate them.
Opeth fan: You just don't get Opeth, they are too complicated for your tastes
person 1 and person 2 proceed to beat the living shit out of the Opeth fan and vow to never listen to anything Opeth ever release ever again
i guess i'm lucky to have known not-annoying opeth fans, because i haven't seen any act this way. i've seen some declare them best band ever, and then someone trolls the opinion, and they troll back. but never inject opeth into a conversation they had nothing to do with, and then get all annoying like that.