It’s going to be devastating for the Dems - for two years they are screaming about fake collusion with Russians, and Mueller report will be nothing more than just another manufactured character assassination a la the Steele dossier.
What more can Mueller say that hasn't already been leaked to the press? They've already tried to destroy him politically, and delegitimize his Presidency these last two years. Hopefully they've thoroughly blown their "politically devastating" wad.