Is frequency analysis the only attack on a simple substitution cipher?
Yes, there are many. Here are the few that I know:


Brute-Force: Simply trying all possible keys.
Kasiski examination: Exploiting the repeated words in the ciphertext to figure out the key length in Vigenere Cipher.
Friedman test aka Kappa Test: Measures the unevenness of the cipher letter frequencies to break the Vigenere cipher.
Key elimination: Eleminate the key by XORing two plaintexts with the same key.