It takes on water, negating it's advantages. The thermostat does regulate the temperature, but only within the limits of the heat exchanger, cooling media, and available airflow. A lot of engines converted to NPG do in fact run hotter.
A lot of classics, however, have seen reduction and stabilization of cooling temps after conversion. This is especially true in engines that overheat due to runaway localized boiling. NPG'S boiling point is so high, it never gets a chance. These engines will typically run hot enough to vaporize any water that is absorbed while sitting.
I thought about it when I built my charger, but went with a 36psi cooling system instead.