I am not sure if there are a lot of testers from central-eastern europe but as i am from slovakia and i am quite educated in history of this part of the world as well as russia i found several historical inaccuracies just seeing the initial screen of this mod - the one you choose your civ from.
1. Kievan Rus leader shouldnt be Jurij Dolgorukyj - actually it was him who founded Moskva/Moscow, so if you want to have different kievan and muscovy civs this is not a good choice even if he was a kievan ruler. there are other even more important rulers - you can choose from a wide range - Rurik, Jaroslav Mudry/The Wise, Sviatoslav... I would definitely choose Jaroslav Mudry/The Wise because under his rule Kievan Rus flourished the most. Rurik is also good as the founder of the Rurikovicej dynasty, but he actually ruled from Novgorod, and he was a Variag- a Viking.
2. Hungarian UP - Power of Tolerance - maybe some of you will acuse me of being anti-hungarian as i am a Slovak, but thats not true. I have nothing against modern Hungarians, actually my gf is partly of hungarian descent. But since the middle ages till the XX. century the non-hungarian parts of hungarian monarchy - such as Slovakia or Romania - was under heavy magyarization/hungarization, people were forced to speak and learn hungarian so the power of tolerance is mosty inaccurate. Again i am friends with hungarians today, but during the Hungarian monarchy its politics with the minorities were quite represive.
3. Russian city names - i would be able to extend the russian names to various prominent cities of historical europe, which were never under russian rule, however russians have their own names for them. As an example - Constantinople should become Carigrad/Tsarigrad after conquest by russia. Its an old russian name for the city, or maybe old slavic as it is still used in some proverbs even in my country.
4. Bulgaria - i dont remember exactly but i know the word khan and khanate is used with bulgaria in this mod, im not sure if they had a khanate sometimes in the history, but they definitely hed tsars for the most of history, so its not good to refer to bulghars as khanate - which would relate them to mongols or others steppe nations.
As I said I didnt even start playing so I may have more comments in the future, comments from the point of view of a slovak, central-eastern european, a slav, a russian history student. But if my comments are too late to incorporated in the game or are not very appreciated, just tell me and i will stop disturbing with such long comments