Pripyat Marshes are not a big stretch of the North European Plain. You can move massive armies through the northern route along the Baltic coast, or the southern route through Ukraine.
They're not really a big obstacle, given the two land invasions that came from the West in the 20th century.
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.
They're not really a big obstacle, given the two land invasions that came from the West in the 20th century.