Current is directly proportional to voltage only if temperature of conductor remains same. If this condition holds, graph b/w voltage and current will be linear. But as we know with passage of current w.r.t to time, temeperature goes on increasing due to which resistance increases and current starts to decrease even with the increase of voltage. This behaviour shows non linear relationship b/w voltage and current.
This law cannot be applied to unilateral networks. A unilateral network has unilateral elements like diode, transistors, etc., which do not have same voltage current relation for both directions of current.