What it does for me is illustrate how little I know about some of these armies. I always though Black Templars were the Space Marine equivalent of Sisters of Battle (not in the back ground of course) but apparently they don't get on. And I'd expected Eldar and Dark Eldar to hate each other as per Trope, and that Dark Elder where on the Chaos side. Gives the Eldar quite a few army lists to draw from.
On a similar vein, the Tau get off pretty well, I guess that's politics for you and not understanding Chaos 'Deamons, oh superstitious monkey, clearly that race has a bad skin condition.'. Also best buds with the Eldar (apparently), I always thought they had it in them to form an alliance (mostly where they would get added if you had to lump them in sides for a soon to be cancelled video game.
Others were more or less as expected, Guard, between the IG themselves, Lost and the Damned, Gue-Vesa, Mercenaries (or inquisitorial plants), can pretty much go anyone. Thought they would have been more in with Inquisition but clearly they have to watch over their shoulder the whole time.
Question remains What would Deathwatches look like? Obviously initially you might think all xeno's would be NBTA, but might their be situations where it might be best to side with one against a more abominable enemy?