All hentai sex is bad hentai sex, if I'm honest.
Really, what I hate is really degrading sex that's not pleasurable or will even be painful for the woman involved. Most futanari works are aimed at men and I really feel that when I read them. They leave me, a woman, wincing and wondering what exactly is supposed to be sexy for me. Sometimes authors throw in a get-out clause like, "for magical reasons, she only feels pleasure/can't experience pain!", but that doesn't suddenly make it sexy to me.
Well, in case men are reading and don't understand... It's like if otaku women for some reason had a fetish for seeing huge rods shoved up men's assholes until it hit their stomachs, but tried to make it "all okay" by throwing in a "he feels no pain, he enjoys this!" Even so, you wouldn't enjoy reading it, right?
I'll go into some examples.
One is really forceful and painful oral sex. There's stuff like when they grab a woman's hair and shove her head down until she chokes, or they just generally place emphasis on how the penis is so far in it makes her gag.
Also treating oral sex as if it gave the woman giving it physical pleasure? I get being turned on by it and appreciating the sensations while you're performing it on someone, but hentai has women moaning and orgasming from it. I just find it lazy, if I'm honest.
There's also just ridiculous stuff like penises so big it would tear a person in two, a weird focus on hitting against the cervix (I know it works for some women, but I don't think it's a common enough desire among women to really expect it to succeed), and even incredibly horrible and impossible stuff like penises breaking through the cervix and going into the uterus or other organs.
I also really dislike the way that written hentai sex scenes focus purely on the physical and the bizarre. Writing has the benefit of being able to describe stuff on the level of every human sense, but they don't take advantage of that.
And they just immediately start treating women like sex dolls, ordering them about, calling them degrading names, so on and so forth...
I know there's a fetish for these sort of things, but I honestly wouldn't mind them as much if they were presented as works designed for that fetish. Instead, they're just thrown into what's supposed to be "normal sex".
Overall, the woman's pleasure is completely ignored. She has to get her pleasure only from providing pleasure, without the slightest understanding of female anatomy, so it's boring and a waste of my time.
TL;DR: Most futanari sex is aimed at men and isn't designed to bring pleasure to me anymore than it is the poor woman they heap themselves on in the story.