[UncleYee]

For what it's worth, on a subsequent turn I did eventually see what I believe was Rage taking effect on an Umbral. An Umbral turned around and started attacking another Umbral, which certainly seems like Rage in action. While there was no icon or status indicator that I could see, I can't think of anything else that would have caused it.

It's possible that Rage had worked other times as well without me noticing it. I figure that if an enemy is affected by Rage, and that enemy is a larger unit type that is pressed against your front lines and adjacent only to your troops, then I imagine that even with Rage he'll still be attacking your troops anyway. Which makes the spell often a wasted cast even if it does take effect.

But anyway, at least now I know Umbrals are not immune.