Seeing isn't enough... you still can't do anything about them. You can only put a wall in front of them or use a crystal, but it would be better to shoot at someone or apply protection to your own units (if it's a Knight) or summon or raise units (Mage, Necromancer). But the complete absence of ranged attacks in an army is a nightmare. And storming a city with just one Chaos unit, which people have started to level up less often... It would be better if they kept ranged attacks, but firing from invisibility is cheating.