I think there was a failure of Russian intelligence to consider that part of the Ukrainian army would defect to the Russian side or lay down their weapons when the war started.The main reason for the Russian underperformance, IMO, other than Western assistance which makes this more a NATO-Russia war than Russia-Ukraine, is that Ukrainian society has completely mobilized from Day 1 whereas Russia has not even partially mobilized.
Whereas the use of Russian troops at the beginning to employ front-line recruits was a giant operational flaw of Russian war planning, this changed even in the first week of the war, but it was a gradual change, as was the much smaller number of troops. On the whole, the Russian advance could have been much greater had it not been planned and executed as it happened, which could certainly have set the stage that the Russians were considering in terms of the Ukrainians' defection and willingness to fight. This was so impactful in the war that Ukraine even managed to get volunteers and mercenaries from all over the world to fight the Russians, so the initial planning of the Russian war was terrible and can be considered an intelligence and operational failure without fear of falling into error of being a flawed argument.