Russia is being tactical in this war. This is not them all out I think. They are withholding some equipment and manpower. What I also think though is that they thought they would be much quicker and encounter less resistance. Modern urban warfare in a fairly developed country with modern expensive equipment (javelins etc) is unpredictable.Mutually assured destruction holds, otherwise we'd all be dead already. Besides, with how poorly the rest of Russia's military equipment has apparently been maintained, it wouldn't surprise me if at least half of their nukes are non-functional. The rest would likely be easy to sabotage in case of real emergency (Putin's cheese slides all the way off his cracker, so to speak).