Recent work on theoretical aspects of steganography resulted in the construction of oracle-based stegosystems. It has been shown that these can be made secure against the steganography equivalents of common cryptographic attacks. In this paper we use methods from complexity theory to investigate the efficiency of sampling from practically relevant types of channels. We show that there are channels that cannot be efficiently used in oracle-based stegosystems. By classifying channels based on their usability for stegosystems, we provide a means to select suitable channels for their practical implementation.