Last I checked, the Germans crushed the Russians in WW1 while treating that front as a defensive action while the real war was fought on the Western front.
The Polish invasion in 1920 was pretty effective too until it wasn't, same for Napoleon, etc. Point being its a big corridor for large troop movements.
Both unsuccessful. But I note your observation that the Pripyat is not as large as my crap geography led me to think.