Warcraft III features quite a few air units. Air units are easier to get and play a bigger role than in Warcraft
II, but in Warcraft III there are many more options for countering large collections of air units. Large
collections of air units are not as overpowering as they were in StarCraft.
Air units advantages
Air units can do hit-and-run attacks. Air units can bypass ground obstacles, get in and attack, and then quickly retreat.
If the enemy doesn't counter the air units using spells or units designed to counter
air units, air units can sometimes be invincible.
Air units can hide in areas of the map where ground units cannot attack them.
Air units can reach islands that are not accessible by land travel.
Air units can kill Creeps or enemy units to gain experience in hit-and-run attacks away from your Heroes.
There are many Hero items that are good against air units that you can find or sometimes buy.
Air Unit Counter Buildings
Halls of the Dead