That's one of my 'jokes' with my Polish half of the family: Poland chose just about the shittiest place to be in: a plane between Prussians, Russians, Austrians...
By the way, don't forget the most important internal divide in Eastern Europe: orthodox vs catholics.
A remark I often make with my family is: Poland is about the optimal spot on the planet! We have mild climate, no weird weather events besides an occasional flood, no earthquakes, volcanos, etc. We have sea access and mountains to scale. We're just about enough developed a country for a comfortable lifestyle. Our crazies, both left-wing and right-wing, are just mildly crazy compared to countries around us. We have enough resources to live off and sell, but nothing so substantial as to induce a resource curse (and in particular, no oil = nobody will come to bring us Freedom). We're not the bleeding edge, but also not the lagging tail. We're comfortably average.
Same for Romania - good climate, access to the Carpathians and the Black Sea. However, I find my people to be pretty pessimistic altough I now believe we actually have it a bit better than we think.
Don't forget the Mongols/Tatars. There were serious raids even in 15th century and Poland's mostly plain and open landscape made things so easy for the raiders!
By the way, don't forget the most important internal divide in Eastern Europe: orthodox vs catholics.