And any captured Demon automatically gains Crazed promotion?.
I have to ask: "Why?"
You've made a difficult promotion to get, with no value against non-demons, for the sole purpose of capturing demons.
Then you want to nerf it into oblivion, by making the demons useless for any longterm plans.
If I wanted disposable units, I wouldn't invest 6 promotions into possibly capturing demons from my enemy. I'd invest 2 promotions into summoning magic, and avoid the drawbacks of exposing the caster to combat, paying upkeep for fodder units, healing cannonfodder and possibly... probably, not facing demons in the first place.
What's next? A promotion that requires Dwarwen, Enchantment I, Combat V, Hero, Drill III and Shock I that allows you to kill any Worker units you defeat?
I'm not overly fond of the Shock and Cover lines either, since they are nigh useless against mixed stacks, and completely useless against single units if your guess was incorrect.
If always capturing demons is to powerful: Either make it unreliable, like Command; or my preference, make it give a combat penalty against demons. Capturing something is always harder than killing it, and it would make demons, your preferred prey, more likely to defend.