If I understand your question, you mean an opamp built using MOSFETs in the subthreshold region?
==> One disadvantage is that because your operating currents are so low, your opamp has a low slew rate (not much current to charge and discharge parasitic capacitances).
Low operating currents mean that you have very low power consumption (which is a good thing) but you also have lower slew rates (which is a bad thing).