"I wonder if anyone has taken a bus to eastern Europe (particularly
from Germany or comparable) and their experiences. Would you recommend
it, and how is such an over 24 hour journey by bus? I know that for
many destinations a discount airline will be much cheaper and much
quicker, but the buses go to a lot of places these airlines don't fly
to, and the prices are fairly low."

Use a discount airline to fly as close as you can, then take a train.

Trains are really cheap in Eastern Europe, so you hardly save anything
by taking a bus.