The "De Medici" reform is just horrible IMO. Whenever you re-elect someone only one of their stats go up instead of all of them and in return you get 25 more of a random monarch power and the ability to select a canidate with random stats... A buff would be nice.

"State sanctioned missionaries" For the Teutonic Order could use a small buff as the +1 free policies is really nice. Maybe a small culture conversion cost or +1 missionary? A buff to zealous administrators would be nice as well as the -10% stability cost is ♥♥♥♥♥♥♥♥♥♥ as a theocracy.

"Tatar mercenaries" could use a buff as 10% cavalry combat ability and 2.5% mercenary discipline is kinda underpowered since the only people that would take the reform for the cavalry combat ability is hordes to stack up the combat ability for cavalry and hordes don't use mercenaries as they have no money and limitless manpower. And Europeans/rich countries that take it for the mercenary discipline would most likely use cannons instead of cavalry. The bonuses just don't make sense for gameplay purposes.

The de-medici reform is balanced around the continued picking of de-medici members, and getting on overage great stats, but increase slowly.
Someone on the team ran a simulation where the result was some 20% increase in monarch points. So it might look and feel bad, its should be stronger.
Also, it's supposed to represent a corrupted oligarchy, ruled by elections primarily in name.

The theocracies are getting a overhaul in 1.3, and this will include changes to that tier.

True, Tatar Mercenaries might not be a broken option, but if you compare it to the alternatives, it gives you raw unit quality without a penalty. So I fear that if the bonuses were slightly stronger, it would be objectively better to pick it over both generic military reforms as well as over any specific ones. Compared to professional army you get half the bonus, without extra maintenance or losing mercs.

In my game England and Castile always end up become Merchant Republic and creating bunch of vassals nations...

That is a very particular bug, ill look into it.

Why does Muscovy's "Oprichhinia" give -10% unjustified demands? When unjustified demands has to do with diplomacy of other nations and not internal things? IMO the reform give a small corruption decay and -5% nobility influence and +5% monthly loyalty.

Question: Empowered Nobles of the Robe offers the Noble Officer Privilege, er, privilege, which says General Skill scales with Land Ownership. Does that mean in addition or instead of Army Tradition? Because it feels more like a downside when combined with all the army tradition boosts the reform and privilege give if it's a replacement.