I LOVED the ending. It's frustrating, sure, but it's perfect for this film: the movie is, at its crux, about a young woman dealing with PTSD and all its side effects, including flashbacks and paranoia; my fave aspect of the entire film is how they never confirmed whether her paranoia was justified, or whether it was her imagination going into overdrive, her inability to 'move on' and leave her past in the past. Like Webslinger said, being told conclusively whether her paranoia was founded or not defeats everything the film accomplished.
For me personally, it was a perfect film. I have PTSD and am so happy to see a movie tackle it so effectively. The past blending into the present, constantly being alert for danger even if it doesn't exist, nightmares vs reality, etc. Another reason why the ending works is that for PTSD, it's most often a lifelong struggle. It's something that plagues people's everyday lives for years and years and years; even if the people behind them weren't cult members, Martha is still going to be living the rest of her life looking over her shoulder and mistaking shadows for monsters. The ending reflects the way most PTSD sufferers live their lives: with fearful uncertainty. Am I safe? It's a question Martha will be asking herself for a long, long time; why should the audience be given an answer to this question when our protagonist will not?