What are the benefits and disadvantages of having multiple religious heads as landed vassals?

Question

Current Condition

• Emperor of the Roman Empire, King of Greece, King of Aquitaine, King of Jerusalem.
• Has The Pope (head of Catholic) as my Vassal, gave him the Duchy of Latium and all the counties in it, thus making the Papacy no longer just a titular title.
• Has The Ecumenical Patriarch (head of Orthodox) as my Vassal, gave him the Duchy of Athens and all the counties in it, thus making the Ecumenical Patriarch no longer just a titular title.

Additional info

• I'm a Catholic and plan to stay so till the end of the game.
• Has Free Investiture. While it give -30 penalty it also gives +10 so it only gives -20 penalty with the Pope, but every other vassals love it.
• All of my vassal kings (including the Pope) are of my dynasty and so is the most of the dukes (90%).
• The Ecumenical Patriarch is not of my dynasty.
• I currently am Greek and plan to be one till the end since I like castrating rebels and seducers.

I was the King of Italy (was Emperor of Holy Roman Emperor at that time with the Pope as my vassal after pushing an Anti-Pope's claim) and then gave it as viceroyalty to my vassal to keep up with my vassal limit. I gave the Duchy of Latium to the Pope during my rule as King of Italy in an attempt to reduce my vassal count and also to make better use of the Pope since his title is a King-tier title.

I conquered Byzantine Empire after I control a successor who has the claim on it. After that I also have the Ecumenical Patriarch as my vassal. Since he is also of King-tier and to give an equal treatment towards both him and the Pope, I gave him the Duchy of Athens.

What I want to do

Yesterday, the Pope declared Crusade for Jerusalem and I managed to get the land after contributing the most in the war. Now, I want to restore the Kohen Gadol (head of Judaism) and land him just like the other two. I know how to do so, so you can skip that in the answer.

My Question

With only one duchy under them I see no problem at all, but what if, let's say, I gave them Duchies with their respective Holy Site? Like, if I were to give Duchy of Galatia, Duchy of Kent, and Duchy of Koln to the Pope.

What are the advantages and disadvantages of having head of religion as vassal with lands? Also since Judaism is a different religion group than Catholic and Orthodox, is there anything I should be careful of with?

Answer 1

The downside is that you're giving land and power to people who will always have a low opinion of you. A landed religious head has:

• -25 from wrong government
• -10 or -30 if they are the wrong religion
• Almost certainly -10 from wrong culture
• -25 if their religious head title is king-level
• No bonus from prestige
• Another penalty from not being on the council if those lands make them a powerful vassal.

Theocratic vassals rarely start trouble, but can still join it and with such a low opinion they will give you very little in tax or levies.

You are almost certainly better off giving those lands to feudal vassals or viceroyals that you can trust.