In the late 60s in San Fransisco there was a serial killer who went by the name Zodiac. His first killings were followed by a three part letter mailed to three different newspapers. Each was a code. Technically, it was a substitution cypher, but for the more common letters, like E and S, he used a rotatating set of symbols. The letter E had 11 different symbols. The letters were sent tothe FBI and naval intelligence and remained unbroken. The uncoded letter that cam with the coded ones said that if the coded letters were not published there would be more killings. Two newspapers published them on the first day, the thind waited until the next day to do it. A man or his wife that liked puzzles sat down with the three papers that Sunday morning when they had finally seen all three parts. Whichever liked puzzles started working on it and the spouse who had no interest in puzzles up until then joined in. They had craked it within 3 hours. the FBI and Naval Intel confirmed that their solution was most likely correct.

A very short message with a rotating set of symbols may be nearly impossible to crack, but I don't think any cypher created by a human would be totally impossible. Someone somewhere will have an "a-ha" moment and the whole thing will open up.

Personally, I've always been partial to doing multiple cyphers. Scramble the letters, using odd words with a minimum of double letters and common strings, then using a box cypher to finish it off.

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I'm not evil.
An evil person would do the things I think up.