Any encryption scheme merely delays and increases the effort needed to read a message. That should be well understood by anyone who uses any encryption scheme. In some ways encryption makes your communications more vulnerable as attention tends to be focused on encrypted messages, rather than the vast number of clear text messages.
Of course the best way to keep your message safe is to use a one time use code, not a repeated cipher.
The other thing that protects your messages is the provision of vast amounts of false information with similar cipher techniques to those used with your true information. To work best, this is done with a plan as to the false ideas you want your enemy to think is true, and the true ideas you want your enemy not to know.
During WWII Germany tried to present an image of great strength, so enemies would be discouraged. They sought to plant the notion that they were manufacturing 1400 tanks a month.
Analysis of a few captured tanks in north Africa put the lie to that. The serial numbers were collected and seemed to all be very close together. Analysis of castings showed that the parts came from a small number of masters, and that put an upper limit on the rate of manufacture.
For people who seek to look behind the lies disseminated by propagandists, there is a good Wikipedia article on “The German Tank Problem”.
Is the Navajo language still available?