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So, I was reading over the rules for the Ravenwing Black Knights, and they state that "If a Character unit from your army with the Leader ability can be attached to an Outrider Squad, it can be attached to this unit instead."

Reading the rules for Suboden Khan, there's nothing specifically tying him to a mandatory White Scars force, so he could be taken with the Ravenwing, giving them access to advance/fall back shoot and charge, which seems amazing with their rule of Anti-Monster/Anti-Vehicle 4+. Plus ignoring terrain when moving too which is always nice.

Anything I'm missing that says you can't do this?

Suboden Khan has the [WHITE SCARS] faction keyword. You can't mix and match Chapter-specific units.

Yeah, I was only looking at generic keywords not faction keywords.