Why is it not suitable to use a soldering gun to solder modem electronics component?

Solder guns as a rule have wider tips than soldering irons/soldering pens.You might not be able to reach just the spot that you want to with a solder gun given the size of the components, and how tightly packed they are on a modern circuit board.

Many modern components, such as integrated circuits, will be damaged by the high heat levels produced by soldering guns.