I probably did not do myself a favor by reading what must be the best entries of this series first.
Also, if I had to hire one medical specialist to take care of my criminal group, an ophthalmologist would be my LAST choice. Come on.
I don't think the Stocklolm worked very well. For one thing, the majority of the book is in Sobek's, the captor, POV. He's already obsessed and self-proclaimed in love with Matteo in the beginning of the book (boring). He spends all his POV telling the readers his whole plan. He tells Matteo his whole plan. Multiples times. It's not a particularly clever plan, too. There's no mindgames. It's very in your face. There's no creepy or spooky atmosphere. This is NOT Shai.
If I were to write out all the things Sobek does to poor Matteo, of course, it would sound very dark and bleak. And the CW are to be taken seriously! But the writing style does matter in setting up a dark tone. And you can write about blood licking or whatever all you want, when the psychological manipulation and torture is so bland, undergone by a MC you don't care about, done by a one-dimensional villain you can't feel connecting with the first MC, it doesn't matter; it's like it's done just to be shocking. And even then, it was predictable and boring, so it was just... whatever. Worse than that, the whole "mindbreak" happens OFF-PAGE! You see Matteo still being combative, and then, next chapter, there's a time jump and suddenly he's all docile and stocklolm-ed. Great. Whatever. This lacked so much subtility. The sex scenes didn't even salvage this for me; they lacked finesse and were repetitive.
If you want mindfuckery, go read Shai instead. If you already read Shai and don't know what to do with yourself now: honestly, same. Tell me if you figure it out.