Exorcism spirits are currently able to target and attack these units:

1. Undying's Zombies
2. Undying's Tombstone
3. Phoenix's Supernova
4. Shadow Shaman's Serpent Wards
5. Weaver's Beetles
6. Tusk's Frozen Sigil
7. Zeus' Nimbus
8. Gyrocopter's Homing Missile
9. Clockwerk's Power Cog
10. Observer Ward
11. Sentry Ward

But since they technically don't attack, they deal no damage to them. This means:

1. The spirits waste time attacking units they can't ever harm.
2. These units can tank a lot of damage, since the spirits can choose them as random targets
3. The attacks done against these units increase the heal count of Exorcism once it expires, which can be abused

There should be a blacklist of units the spirits can't attack.