Are there really that many M’s on NU?

No, if you say you don’t like it after 5 chapters I’ll say okay, don’t torture yourself. I might say the tone changes at
Chapter 100, but who wants to be miserable for 94 chapters between? I wouldn’t want to read those chapters, and I wouldn't do it. Too many other books out there.

But I can see that others might say keep reading. 

Hating on things is pleasurable. I think the English expression is  "love to hate"

Theres a hypocrisy in what you're saying.
If I read 5 chapters of a book and say "it sucks , screw this book" , you'd say "aww you didn't read till the good part" if you're a fan.

So for your sake and out of respect , I'll go and read it until it pisses me off completely.
When that happens I have completely 0 guilt when I'm dog-piling on it and no regrets telling you that you enjoy shit.
On top of that you have the full knowledge of what happens in the book , so you can argue every point. 

They probably were hoping that it gets better 

they're tsundere. they're actually loving it or at least care about it or they won't be wasting their time. 

You have to keep in mind that the only people that typically speak out really loudly are really on the extreme ends... either they really love something or hate something.

Kinda like this:
  ~~  ~~

People who are neutral or just close to the middle in their opinion don't have nearly as much motivation to complain or praise something, why would you rant about a thing you only kind of care about? So it seems like it's a lot of people, but really it's a vocal minority.