The units Ancestors creates are fairly capable at fighting, but will only last one minute before dying. In addition, they can be easily defeated by an organized force. As such, they should not be used as a primary fighting force. Rather, they should be used to support the player's own troops during an attack, or defend against a surprise attack.
As Minions are myth units, Isis players may consider invoking the Eclipse god power shortly after invoking Ancestors. This will increase the Minion's strength and prevent the enemy from using their own god powers to counter them. Ceasefire is the best counter to Ancestors as the Minions will automatically die shortly before the battle can resume.