To be really pedantic, ALL of them are ultimately air cooled. The only question is whether the heat goes from the CPU directly through a piece of metal to reach that air, or if it hitches a ride on some liquid along the way between two pieces of metal, one at each end of its journey.
-Pat
Thank you, and I did say that air is used in both system as the final means of cooling, but one of them actually dumps the warm air back in the case and then relies on another fan to pull that air out of the case, which is way less efficient than having a rad mounted at the very edge of the case and then blow cool air through the rad to so the warm air is then dumped outside of the case.
And the few hundred Watts dumped from the generally Air cooled GPU goes where?
Well all the air cooled GPU's I've seen (I'm assuming that you're referring to those with cooling fans rather than passive ones), all suck cool air from inside the case and blow that air passed the GPU and other chips in ductwork straight to the exhaust grill, which normally uses the expansion port either above or below the bracket the ports are located on, and the air dumped once again outside the case.
Really 'straight to the exhaust grill'? In the fairly common case of a Tower the GPU (the dominant power emitter/consumer in a gaming/work rig) the only reason that heat goes out at all is because of airflow from inlet fans and these days likely an extra fan at the rear.
If there is no fan at the rear grill the heat from the GPU will take the path of least resistance and go straight into the fans (area of least case pressure) of your likely top mounted Radiator decreasing cooling efficiency unless the case is positively pressurized from the front fans. Even then some of that heat and airflow will likely go into the Radiator.
To much rear fan and you are back into losing one of the so called benefits of water cooling in lower noise and also the reduction in internal case pressure will reduce airflow across the AIO or the fans will need to spin harder to maintain temperature. So it is anairflow juggle.
In the other fairly common case of a front mounted AIO the air from it does not go straight to the outside either and relies heavily on airflow to keep the box cool so in this instance the heated air does not dump straight out at all. So you can not make a general claim about heat (CPU) going straight from the case when you have water cooling it is case and mounting specific.
And none of this is KISS it is additional airflow complexity and more $ spent for a marginal perceived gain.