Sorry. We throw that term around a lot. lol.
It's actually not a very accurate term... should probably say "power mosfet".... MOSFET is just a type of semi-conductor technology. A power-mosfet is a switch.
Think about the Hoover Dam. It has big gates for the water flow.... So, do you think that someone goes out to the dam and turns a REALLY BIG wheel to open and close the gate? Probably not. They put a motor on them, and have a small lever in the "control room" that turns the motor on/off. So a little switch controls a "thing" that moves a bigger thing.
That's why we use mosfets in mods... little buttons. The little buttons (like on the eGo) would MELT/break if all the electricity went through them. A mosfet is a switch that is turned on by a smaller switch. The electricity goes through the mosfet, not the button (well, most of it).
Even the power mosfets have ratings as to how much electricity they can take before they overheat and fry.
As far as DC's.... the 3.7 volt mods should be fine, I would think. The eGO stuff... may depend on the ohms and the vaping style... but risk it if you wish. I've seen warnings, but I'm not sure for what ohms. I think MV had some warnings for certain ones. The all-mechanical mods in particular should survive it, since they don't have a mosfet.