I'm not saying things couldn't be even better, but looking at Google Maps, I could still reach Bucharest from my ex-eastern block country capital (Prague) via highways with just a short stretch of non-highway road in Romania (I'm actually considering the trip, I've never been to Romania).
You're right about the former soviet republics though, the trip to Riga and especially Tallinn doesn't look very comfortable.
Just for info, after you enter Romania via Arad you have another 350km of highway, then a 170km break (mountains, bad traffic) and another 100km before Bucharest. 600km that you can drive in 4.5 hours on highway takes over 8 hours. If you enter via Oradea, it takes even longer. 15 km in Bucharest at rush hour takes 2 hours. If you want to come, you are welcome, there are some nice places.
You're right about the former soviet republics though, the trip to Riga and especially Tallinn doesn't look very comfortable.