The Only Ones is your average mother-daugther story, except the setting is a post-apocalyptic wasteland ravaged by disease and the daughter is a clone. I know you're thinking, thanks for the spoiler, but these details are all established very early in the story. I'm not giving too much away here.
Surprisingly, what works best about this story is the mind of the mother. She's a little slow at times. She's also extremely repetitive (“I will tell you this,” “maybe you are thinking,” “I don't even have to,” “I just,” “I must of,” “it's all I can...”, etc, etc.); it's neverending. Aside from her quirks, Inez is in many ways a typical mother with typical worries who has a way of earning the reader's sympathies. If she were someone you met in your daily life, you'd likely find her irritating or far from interesting, but given a window into her mind, she becomes your very own mother, the good and the bad. It is this universal mother that shapes this story and gives it all of its strengths. Sure there are raiders and philosophical questions about life and pages full of scientific explanation, as there are many intense moments, yet at its heart, The Only Ones is a story of a mother's heart. It's as simple as that.