TL;DR The book is full of decent and intriguing ideas that are poorly executed. There is no character for the audience to use as an anchor. It takes 160 pages for anything resembling a story to get started.
This book is a surprise and mystery if you have never read metaphysical horror before. To see what Annihilation would look like if executed flawlessly, read House of Leaves. The now-commonplace horror elements of humans encountering the utterly foreign and unknowable make up the backbone of the narrative. But whereas it is on glorious and staggering display in Lovecraft, King, amd other giants, here it is reduced to a bizarre oddity that induces head-scratching instead of spine-tingling.
The primary reason for this is the clashing styles. The first 160 pages of the book are so sterile and cold that even the strangest things are as interesting as unfinished jigsaw puzzles. The main character, The Biologist, states that this is to provide an objective account of events which necessotate a subjective experience in order to terrify. The last 30 pages are forced to give up the objective description, and only then does Annihilation actually become interesting.
The Biologist herself is done great injustice by the first 160 pages. In providing nothing but objective descriptions of events, the author fails to establish her as anything resembling a human being. She is nothing more than a camera lens that requires occasional flashbacks by the author to establish that she has Emotions and these are often at odds with the character as portrayed. The Biologist is a heavy introvert. She eschews people for her work. This is mentioned often and in bold fashion. Yet this person married, for reasons that are never explained beyond "he offsets my introversion". The Biologist is not portrayed as a person interested in the feelings or experiences of others. Why marry?
At the end of the book the semblance of a person begins to emerge from the Biologist, but by then the narrative has established her so firmly as a non-presence that her character feels like something the author shoved in when he realized there was no point to which the audience could attach. She is a walking fight between a detached third-person objective lens and a woman who wants to tell her own story. It does not end well for either party.
Finally, the book simply starts. No world-building or character history or any point of reference that would be helpful for an audience seeking a way into the story. In theory this plays along with the idea that the characters themselves know little, and are themselves poorly informed. In practice it is disorienting and dull. We know nothing about the state of the world. No baseline is established. Even the characters know more than we do, as is revealed later in the book - well past halfway in. The effect is like waking up to find yourself weightless inside an empty sphere. There is no point of reference and you can only make guesses until someone pops in and tells you what is happening.
There are many more small details that add up into a large pile of errors - debunked pop psychology from 1950 paired with hard science, characters that serve no purpose and go nowhere, jarring switches between clinical observation and surrealist prose, a world that is somehow both tantalizingly alien and horribly mundane - but describing those would take much longer.
Read a summary on Wikipedia. I guarantee it will be much more cohesive, interesting, and above all much less time-consuming than reading this book.