i was writing about this since Old Gods expansion - slavic religion is by far the worse and the most boring in the game (with possibly the Zunists that I've never played)
Romuva, African and Suomnsko are brainlessly easy to reform, and two of them now have the Eldreship succession, helping you to keep you Empire united.
Germanic have a ton of flavor, events, stupid bonuses (how is to navigate rivers more complicated to navigate seas???)
And as Slavic, you have your vanilla pagan religion without any interesting things, you have to conquer an anhistorical and gigantic empire to reform (or making sneaky conquest making your map bleed your eyes). Your neighbourgs are overpowered Tengri hordes, that are immune to which is supposed to be your biggest strengths (defensive attrition) And you're locked in Gavelkind. Have fun.
I've never played any slavic character except to convert asap, which makes like 30% of the map implayable for me in earlier starts.
Reformation as it is have absolutely no sens, and is boring to play. It should be character related - be independant, have <20 learning, have a temple vassal with 15>Learning, have 500 piety. It should not be instant, instead it should trigger a period of instability (malus to relationship with your vassals and give a special CB on you to your same religion neighbourg for 5-10 years). Or perhaps even several wave of negative events, a bit like westernization in EU3.
And if you manage to survive your reformation period, nobody outside your realm should convert to the reformed religion. That mechanic would make far more sense to me.