LadyKCH
Well, I think there are two situations that are greatly different but can both be resumed by calling the personalities of the harem female bland. And you only focused on 1 of them.
The first is, like you said, plain, old and same boring stuff that is just a random name picked here and there. A woman that has strictly nothing to bring to the story, not even some good sentences here and there and just is present to say "Look, the MC has another member in his harem." But if it was just someone here and there, it could have been boring to read, but relatively fine. You spot them quite easily, and you just make up your mind to erase their existence the moment they are gone.
However, there is a second situation. Which really, really, really piss off most readers. The 200 IQ character that suddenly lose 90% of her intelligence when entering the harem. Like seriously, that is the worst thing that could happen. Especially when that heroine escaped the harem for a long time and survived a few of her good sisters. At that point, it is like "Doesn't matter Lu, Fu and Xu fell for the MC, at least there is still Zu, the good Zu." And then BAM! No more Zu.
During all this time, Zu was the equal of the MC. She was saved by him most of the times, but she saved him once or twice. She lost most of their fights, but she still won a few. When there was a competition she was second when he was first. But then, the author decided that since she was popular, she had to enter the harem. And after that, Zu was never the same. She couldn't qualify to enter the competition, at the first problem she had to call the MC. She was suddenly kidnapped but couldn't escape by herself (thing she did a lot of times before). That's when the "bland" personality kicks in. And a horrible one.
And that's without talking about all the love/rape, the dual cultivations bullshit, the trap in a motel by drugging the MC ect ect. But for me, the moment the female second MC is no longer considered as a MC but as love interest, that's the moment I drop the novel. Each time there is that slight hope that it won't be the case, but each time, it doesn't matter if it takes 100 chapters or 500, it always comes. And it is a bit sad.