Workers (And I assume every unit with 0 strength), are no longer valid targets for assassination attempts. This is to make the Guardsman promotion valid (as previously, people used to carry giant stacks of workers, so that the assassins just pick the workers off)
Hyborem is a Hero, and as a result, gets a massive "Decreased chance to defend the stack" (Which I *think* means that the base strength of Hyborem is reduced to a HALF)
Unfortunatly, this means that he, when he is strong-ish, he acts as a decent guardsman unit, even without guardsman, as he draws the assassins to him (!!)
I killed Basium 3 times with that fact (And with the Nether Blade, of course)
As for not picking Imps... I dunno... thats odd.
And ritualists can't see those types of units, but I am not sure that the AI doesn't check to see if it can actually see the unit, when it takes into consideration when to use the Ring of Flames ability.
However, that said, Ring of Flames doesn't kill, just damages. Your shadows may be weak, but unless there is a way it gets detected, it won't die.