[DGDobrev]

They suck a portion of the life of the victim and replenish their stacks. The problem is that replenishing has no limit, so in the end, they may outgrow their number seriously. This means that even if you kill the boss, you will need to kill your own extra troops before finishing the battle.

That is very inconvenient and also one of the reasons I don't use ghosts. They're great, no argue there, but if they outgrow their leadership, the AI stops targeting them (even with mass attacks!), but they will keep on targeting the AI units and will keep on growing. In the end, you may end up with a force you can't kill.