Even if we assume that Gods can communicate with the individual through dreams, we rely on humans (often religious figures) to tell us which dreams were from God and what they mean. This leaves the dream vulnerable to the individual's interpretation.
If some dreams are sent from God but some are not, how can the individual know which dreams are the word of God and which are not? The religious figure has little more than personal speculation to decide which come from God and which do not. This is fallible.