It is in the North, and especially west of the Dnieper River that Russia has performed poorly and that is because the plan was poor. It insisted on over rushed execution.I always believed that the Ukrainians and the Russians were comparable as warriors on a man-to-man basis.
Moreover, the Ukrainians would be more motivated to fight in defending their homeland than the Russians would be in invading it.
Until recently, the apparent overwhelming majority of writers here were expecting an easy (or at least an inevitable) Russian victory,
which would result in the Russian conquest of most of Ukraine. Please think twice before casting the first stone at Russian hubris.
Many or most of those who thought of a quick Russian victory are probably not well versed in the Chechen Wars or they forgot about the Chechen Wars, which are highly applicable to Ukraine, as they are the most recent extensive land wars that the Russians have fought. The Northern Operation by the Russians, especially the northwestern one is in many ways akin to the First Battle of Grozny and the First Chechen War, in which the Russians expected a swift victory and the Russians entered in Grozny with armoured columns unprotected by infantry and air support, but were then decimated as a consequence. Even though Russia did rally and win that battle of Grozny, it came at a huge cost and eventually Russia withdrew from Chechnya.
In the second Chechen War, the Russians came with more than twice as many military units and it took them more than 4 months to capture Grozny. The Russian ground forces advance was slow and deliberate, employing protective measures, fanning out infantry against ambushes, and never rushing combat units far ahead of supplies. It took more than 2 months after the start of the war for the Russians to actually storm Grozny. The humiliation of the first Chechen War had made the Russians much more respectful of the Chechens and as such they were cautious. Russia did eventually win that war.
I believe that Russia will ultimately achieve its objective of expelling Ukrainian Forces from the Donbass Region and it will retain the landbrige that it had formed from the Donbass to Crimea and it will be able to defend those locations from Ukrainian encounters, but the debacle of the Kiev offensive would have been avoided had the Russians applied the same operational and tactical methodologies of the 2nd Chechen War in their approach towards Kiev. Obviously, they never committed the number of personnel to surround or even take Kiev, but that would also mean that they would have negated the hubris of the speed rush decapitation strike on Kiev, and wouldn't have advanced as far as they did, probably stopping at Invankiv. It would mean much fewer casualty and equipment losses, but still enough of an approach to be deemed a feint capable of threatening Kiev with a speed rush at any time.