Poirot and Hastings take on a global crime syndicate in this adventure, which is pretty different from their usual milieu of inheritance murders and society thefts. It’s too big a narrative for these two, and the strain does show a bit.
Suspending one’s disbelief can only go so far. Poirot seems to be not just an analytical genius, but positively omniscient here, as well as ridiculously wealthy; Hastings is somehow both cleverer and dumber than usual.
That being said, I’m pretty sure that Ian Fleming borrowed some of these plot points for his James Bond novels: super-villains out to conquer the world; a master of disguise; a hideout inside a hollowed-out mountain; an atomic threat to global security.
Overall, I’d say this was not Poirot’s finest adventure.