Harbones -
OK - He is talking about the radiator fan....it pulls fresh air through the radiator to cool the engine...
A mechanical fan turns when the engine turns (A drive belt...)...so the faster the engine is running, the faster the fan blades spin, and the more air is pulled through...in addition to the air moving through the radiator from the truck going foward through the air...
When the truck is idling...and the engine rpm is slow, the fan isn't pulling much air....and it might overheat.
An electric fan pulls at the same speed, no matter what the engine is doing...so it pulls more air at slow/idle speeds...keeping the engine cooler.
An electric fan can also be dash switched, so you can turn it off on water crossings to avoid underhood splashing, etc.
Some people mount a (Pusher) electric fan on the opposite side of the mechanical (Puller) fan...so they get the benefits of both.
An electric fan w/o the mechanical fan can save gas/power too...because the drive belt power is not being drained by the mech fan anymore...
If the Elec. fan set up is sized/geared/bladed properly...there is no trade off...Cooling-wise - its all good. ...There is of course (some) extra load on the battery/alternator....a factor to worry about if you are stock alternator equipped/overloaded with offroad lights, winches, onboard coolers, airtools, etc.....
The problems, if they are to occur, are typically due to the fan blade design...pitch or variable pitch, swept area vs shroud overlap, etc....assuming the unit is designed for your application at all...They typically size them based upon horsepower of the intended engine to be cooled, and a diameter it has to fit within.
They cannot account for all of the rig specific variables, such as obstructions to airflow on the engine side of the flow preventing/hindering cross ventilation...or to the exterior, such as a winch blocking/hindering the intake air....so there is some trial and error involved.
The hardest trial is typically a hillclimb, or pulling a heavy load, etc...where you are working hard, under load, which makes the most heat, and going slowly, which limits the rig speed related airflow as well. (A condition where you have - The most heat to dissipate, combined with the least air to carry away the heat)
Does this help make sense of the discussions above at all?
