This question has come up before. I'm not certain there's an answer that will satisfy anyone.
I've come to the conclusion the Faction keywords are the same and units from both Codexes could be included in a single detachment. But I can't think of a single reason anyone would want to sacrifice Legion Traits / Daemonic Traits to do so. It's almost universally to someone's disadvantage to mix Daemons and CSMs this way.
Sure, it would be cool to have a Bloodthirster in a World Eaters detachment, but now none of those models get extra attacks when they charge. A patrol detachment on the side along with a couple squads of Bloodletters would allow those units to deep strike, and that's a much better tactical option.
So, if anyone wants to argue about keywords, tell them go right ahead. I'm positive someone will come up with an edge case I haven't thought of, but that's not going to break the game.
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