The problem with rape and blackmail in NTR is that it can be done well. But its usually just a cop out for doing any actual writing. Its just so often used as a way to have the girl fall right away. "Oh no you saw me kiss my boyfriend at work, now I have to suck your dick in public". Its almost always flimsy as shit blackmail too. Rape works a little better since the "dick so good" angle has some logic to it. However, games that resort to doing that right away almost always have the girl whoring out to dozens of guys right away.
I think most people hate those methods because it's used so often as a shortcut in boring slop.
And ive seen people in every genre crashout when the story deviates from their ideal, This isn't an NTR specific thing, lol.