BD, you're right about the warheads of today being extremely difficult to coax into going critical or supercritical on cue. First, processing the component materials to a level of purity is almost impossible. Then, the mechanics of critical assembly became Swiss watchmaking, just a figure of speech, when America completed minification of payloads. Now, clusters of small warheads could be combined as one large payload or many separately targetable ones.
The process has been stolen, copied, and prayed over, that they would work as designed but most will fizzle as high explosive triggers. There is still the brute force, simpler, but extremely destructive warheads that have a high success rate.
“War is an ugly thing but not the ugliest of things; the decayed and degraded state of moral and patriotic feelings which thinks that nothing is worth war is much worse.” —John Stuart Mill