op amp subthreshold
In the weak inversion gm~Id and in strong inversion gm~√Id, which means that gm/Id ratio is better for weak inversion, but gm in strong inversion has higher value.
Input stage of differential amplifier in weak inversion is able to compress offset caused by mismatch in the drain. Because of exponential characteristics (same as bipolars) offset is smaller. As previously noted current mismatch could be higher.
For weak inversion you need Vgs-Vth~100-200mV, which could be made increasing W/L ratio and decreasing current Id, which is generally slow.