The Banshee in WarCraft 3: Reign of Chaos has the ability to possess units which are controlled by the enemy, or are Neutral. This ability give control to the person who used the Banshee on the unit, similar to the Dark Archon's Mind Control from StarCraft: Brood Wars.
However, unlike in StarCraft, when the Banshee uses its ability on a worker from another race, (peasant, peon, wisp) and uses that worker to build a "Town Hall" for that race, there isn't another "Supply Count" thing, and thus one is only limited to a 90(100) supply count, unless one uses a "Break Supply" cheat, or kills a few units, trains others in their place, and then "resurrects" the dead ones, or "rescues/recruits" a friendly force. Basically, one can't have an army of 100 Undead, 100 Alliance, 100 Horde, and 100 Night Elves to help them fight their foes, unless the other 300 troops belong to their "Allies" (Computer/Player) and have "Shared Control" on.
Now, do you think that Blizzard should have allowed additional "Supply Counts" for the Undead users who controlled Troops from the other races, or do you think that would cause too much of an Imbalance and been unfair to those who played the other races?