What is a major limitation of substitution ciphers?

The major limitation of substitution ciphers lies in their ability to maintain letter frequency patterns. In substitution ciphers, each letter in the plaintext is replaced with another letter, but the overall frequency of letters remains unchanged. This property can be exploited by attackers using frequency analysis, where they analyze the frequency of letters in the ciphertext and compare it to the expected frequency of letters in the language. For example, the letter 'E' is the most commonly used letter in English, so if a specific symbol appears frequently in the ciphertext, it may be a good candidate for 'E'.

Since substitution ciphers do not obscure these frequency patterns effectively, they are vulnerable to cryptanalysis. This makes them much less secure compared to modern encryption techniques that employ more complex algorithms that don't exhibit clear frequency relationships, thus providing a higher level of security.

In contrast, the other options refer to misconceptions about substitution ciphers: they are generally not overly complex to implement; modern systems may still use various forms of substitution in more sophisticated ways; and while encryption times vary, substitution ciphers are relatively fast compared to many other encryption methods, especially when implemented with simple mechanisms.