That is a tough question.
Am I allowed to write a review about a book that I didn’t finish? In my humble opinion? Bloody hell, yes! Especially if I can point out reasons why I didn’t like the part of the book I read. That doesn’t mean I’m slanting the author or the book or that I want to prevent other people from reading the book. I’m just talking about the things that triggered my not liking it; I’m pre-warning fellow readers who might have issues with the same things.
Would ploughing through and eventually finishing such a book change my mind about it? I want to write no, it wouldn’t change my mind, but I cannot do that. Nevertheless I am fairly certain that in more than 90% of my DNF cases, my review and rating would be similar -if not worse- had I read the whole book.
To keep this short. I’ll continue to write reviews about books that I haven’t finished reading. It’s my free time that I am spending on books and I am writing my thoughts about them to not only encourage people to read a certain book, but also to offer critical views and/or content warnings about books that I didn’t enjoy. And if you are a bit like me, then my DNF reviews might actually encourage you to pick up the books and read them to form your own opinion.