I did not fight many undead until now, but I assume they are immune against the stun.
I’m pretty sure I’ve successfully stunned a Fallen Hero before.
If I’m not mistaken, only Orc Warriors and Warlords are immune to stun.
What? On top of knockback immune, they’re also immune to stun? That’s rather extreme. All those stun based skills are more worthless then I thought. After all, the orcs are the hardest things on the map