I don't think people are misunderstanding what the story is about, though. Most of us knew from chapter 1 that this wasn't going to be a healthy romance. A guy biting someone's tongue off isn't exactly subtle foreshadowing...
The criticism isn't that the story is dark or problematic. It's that being dark and problematic doesn't automatically make something interesting or "well-executed". You can portray obsession, coercion, and abuse while still developing a unique dynamic between the characters.
It's also obvious the author isn't romanticizing a lot of these moments and wants them to feel disturbing. The issue is that after the strong setup, the relationship started feeling less unique and more reliant on tropes that are already very common in BL with the huge power imbalance and all, plus the focus shifting from exploring the psychological tension and complexity of their relationship toward using escalation and shock as the only way to move the dynamic forward, at the expense of deeper exploration.
Basically, understanding what the story is trying to do and thinking it executes it well are two different things. Just because a story is dark, disturbing, or intentionally uncomfortable doesn't automatically make it compelling. And people criticizing how the story chooses to portray those themes, or feeling that the story didn't fully deliver on the potential of its premise, aren't necessarily misunderstanding it— we may simply think the execution didn't live up to the premise.