Today's AI can crack second world war Enigma code 'in short order', experts say

if you rely on substitution only, your doing it wrong. the cypher or text is subject to alternate rules of grammar, syntax, spelling, euphemisms, and hardening rules, such as not repeating words, dropping vowels or other frequent characters, intentional misspelling, and interleaving of red herrings.

also:

https://en.wikipedia.org/wiki/Code_talker