What happens to a roof/attic that only has intake ventilation?

Without a balanced amount of intake and exhaust vents the attic airflow will be inefficient.

If an attic only has intake vents, the windward intake vents will be intake and the leeward intake vents will be exhaust. One of the problems with this situation is that the while the attic floor is getting ventilated, the upper portion of the attic where the moisture and hot air rise, is unventilated. Some attics will show these effects in the winter when frost from condensation will form in the upper part of the attic, but the lower areas will remain clear. Without a balanced amount of intake and exhaust vents the attic airflow will be inefficient.