I would like to ask 2 questions.
1. Does this mod allow faction leaders to call all his peer vassals against a liege when rising up? CK2 vanilla removed this feature about 2 years ago and I quit the game completely ever since because the difficulty simply evaporated when playing as a king or emperor, as you can do whatever you want and crush faction rebellions separately one by one.
For CK2Plus, although the vanilla factions (I.E. Increase Council Power/Install Claimant/Change Succession Type) function as they do in current Vanilla, four additional factions (Prosperity, Glory, Tradition, Court) are added that will ask all vassals to join their faction before issuing their demand. In that sense, it works much like the old system, but you do get to know who will and who won't be made up of the revolt.
Also, because it's been two years, perhaps you missed the vanilla update (released with Charlemagne DLC, I believe) where faction revolts are placed into one independent state (called "<Liege Title Adjective> Revolt"). This change really helped improve the AI's ability to combat the liege's troops as the AI acts much more cohesively.
I would also say that factions have become more threatening in vanilla as well because "Increase Council Power" is universally popular among vassals. In addition to that, I think the factions are a much more interactive method of rebellion compared to calling vassals to war because it allows for a more informative experience of internal conflict, both for the rebels (I know who will actually fight with me) and the ruler (I know who to reconcile with or undermine).
For large empires in CK2Plus, there's also an "Imperial Decadence" modifier that imposes penalties on Emperors (Reduced Taxes, Army Morale, Levy Size). This makes the AI Empires in game to rarely expand very far. In my experience though, it hasn't been difficult to keep the decadence level down.
Finally, I'm not sure what you mean about internal defensive pacts. Do you mean that vassals can't fight each other? Foreign countries?