There you have it. I was searching for that through the forum, but didn't find it. So what happens if I have only Elves in play and the Shadow Player plays 
Ulaire Nertea, Dark Horseman and chooses the race "Elf", can I still assign him by using 
Crashing Cavalry?
The answer might be really obvious, but I'm not really sure about that one.
And another example, what happens if I wanna assign 
The Balrog, Demon of Might to any Elf with strength 6 or less? 
I'm not sure if both examples fall into the same category, as 
Demon of Might says: "cannot be assigned" and 
Dark Horseman says "The Free People player cannot assign".