Populist Factions are the more dangerous type of county faction and form when counties wish to be governed by a ruler of their own culture and/or religion.
Did people in the Middle Ages really gave a crap about this? Religion, yes, but culture/nationalism? Isn't that a thing much later in history? Being ruled by a foreigner was more of a rule than an exception in some regions. Of course, difference in culture does not help in a conflict, so the modifiers that also exist in CK2 make sense, but was it really the cause of any European medieval war to 'fight for your own country' and that 'a ruler should be from my culture'?