I think the problem was that they were not flexible. There's a really great book called "Maneuver Warfare Handbook" that goes into the details of this - but the main things that I feel really bogged the Russians down was the poor command, control & comms, poor logistics deep into enemy territory.
Also their failure of hitting their strength against the Ukrainian strength(as it is now - attrition) instead of hitting their strength against the Ukrainian weakness(bypassing areas like Chernihiv or Sumy, where entire Ukrainian divisions were prepared with ATGMs etc)
Manpower was also a detriment. Even the Soviets took double the amount of men into Czhekoslovakia as compared to Ukraine, a country with 8 times less the area.
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If they are to improve their performance this season, it is clear they can't exactly continue this attrition warfare without killing a lot of their own people as well - unless the political and military command is OK with that.