The entire book was a footrace of annoying characters and plot developments. Christie loves her heroine Tuppence, a shamelessly shallow and obnoxiously vain hustler, but her twee demeanor is frustrating rather than endearing. Her would-be lover, Tommy, is oddly aloof and creepily jealous of anyone who comes between him and his frenemy. When they finally get together at the end, it's eye-poppingly forced, and feels like Christie is just giving up rather than concluding things.
But even more agonizing than Christie's leads is her storytelling formula. "The Secret Adversary" is a break from her trademark whodunnit style, with no real question ever lingering about who the bad guy really is -- making the Bond villain-like monologue unnecessary. There is no mystery here involving anything to do with the plot. The real wonder is how an effort this haphazard from a legendary writer could be lumped in with her classic creations.
The one high point is the outrageous use of inventive slang. I wonder whether characters from the 20s really spoke like this, but suspect Christie was just making things up and forcing her characters to talk the way she wishes they would. Which is just what a writer should do.