If you look at the new table of modifiers for Rituals (found in the Tome of Excess), you'll see that Dark Apostles get a -30 penalty towards all rituals dedicated to any god they are not aligned towards. It doesn't say towards gods in opposition to their alignment - which would be fine - just those they are not aligned with.
This means that they have an Archetype power which exists soley to mitigate this heafty weakness hidden away on a table. So the priests of Chaos Undivided (which many of the Dark Apostles would be) are inferior ritualists to just about every other Heretic. Lame.