There exist a number of devices whose OpenGLES drivers do not support SSBOs or image uniforms in vertex and fragment stages.

The current Compatibility mode proposal states that the maxStorageBuffersPerShaderStage limit should be
min(GL_MAX_SHADER_STORAGE_BUFFER_BINDINGS, GL_MAX_VERTEX_SHADER_STORAGE_BLOCKS, GL_MAX_FRAGMENT_SHADER_STORAGE_BLOCKS, GL_MAX_COMPUTE_SHADER_STORAGE_BLOCKS)

This would be zero on those devices, preventing the use of storage buffers at all (even in compute).

Similarly, maxStorageTexturesPerShaderStage would have to be zero on those devices.