I must say, I relate to those who have expressed being put off or dismayed by the "bad tactics" employed in the film. Not only by the primary characters, but the entire operation.
The film Arrival begins with a very similar sort of trajectory. An unknown extraterrestrial event occurs, a specialist is brought in after other approaches have been tried and failed to solve anything. This specialist is part of a team of scientists and military types with a variety of specialties of their own. They enter the anomaly and take a methodical approach in investigating it. Like scientists would. I found it to be a very believable in how such an event would be handled.
In comparison I would call Annihilation frustratingly vague in that regard. The composition of the team is awfully spontaneous and for such a monumental event. Decided, on a whim, by one burned out shrink. "Let's try and all female team because we haven't tried that yet"... but what's the scientific justification in favour of such a composition? What's information points to that being a more compelling option than, say, a team of 5 brunettes and a basset hound? Or a team of 2 people, or a team of 500 people? I may have missed the answer to that question somewhere in the film if there was one.
Suffice it to say, that aspect of the film was all a bit loose for me. Perhaps it could all be explained, if given the benefit of the doubt, but I guess the film hadn't earned that much goodwill from me yet.
I did enjoy the mysterious anomaly though. How it first appears, how it looks, how it grows, what happens when you send stuff in it, and what happens when we ourselves eventually enter it. It was a bit like the spore-world depicted in Nausicaa... or the early stages thereof. The fact that fungi and mold breathe oxygen (like humans) may be some kind of clue about the nature of the anomaly, as that's what seems to be gradually proliferating within the thing. Then again, there are those plant mannequins, and people become bear-monsters, and everything is screwed up, so maybe it's nothing. I definitely got lost trying to understand or speculate about what was going on. The ending was like "okay, a weird mirroring thing", which was fine I guess, then everything goes up in flames, Portman is in a hospital with her husband, and their eyes are twinkling meaning they are infected or changed I guess. I don't know if it ever "made sense" but I definitely stopped trying to understand it after a while.
Flashbacks to Portman in quarantine giving a debriefing were an unwelcome and fruitless interruption I thought. I also didn't see the purpose of flashing back to Portman's interlude with her work colleague. There was a lot of jumping in and out of the primary story... I didn't always see the relevance or how the timing was appropriate, except perhaps to catch our breath after some action. Mostly it just confused matters imo.
There's lots of higher level details and mysteries to speculate on and investigate further, but I don't think the underlying film was stimulating enough or entertaining enough, or just enough, to make me want to come back and do so.