It came to my notice that I assigned a 4 piety governor in 1 province and a 4 dread in another and both had no vices effecting happiness. Funny thing I noticed is the 1 with 4 piety got the bigger loyalty bonus. The reason I noticed this I figured I would do a test to see how much of a difference the 2 made on 2 newly conquered province. When I did this I cleared all units out of the territory and then assigned the new governor to see what would happen. The one getting the dread governor actually had the loyalty drop a bit (if you don't know when yopu assign a new governor to a territory for some reason the loyalty drops almost always and occasionally on odd occurances the loyalty goes up probably usually due to vices) The 1 with 4 peity actually increased the loyalty even though he was a new governor(guess dread has a better long term effect as hes governor longer the more they respect his dread).
Anyways I guess since it was mostly catholic and playing as a catholic faction made that difference but maybe if he was thrown into an all muslim province the effects could have been quite the opposite.

This makes me wonder if a piety governor is a much better governor as long the province matches your religion and 1 that has the opposite religion would do much better with a dreadlord