Its not the Ascians plural, its just Emet. None of the others have been stated to try. And we know Elidibus surely didn't.
And even then its never stated he "worked with the Sundered" in the sense of actually explaining the situation to them, just that Emet tried to see if they could be comparable in his biased eyes while still actively working toward his goals even as he grew to love his First Born mortal son.
That's a really, really generous interpretation of "working with them". You can't "work" with someone you fully intend to betray at a later date and call it giving it an honest try.