More details on the failure of Russian operations to date

 Institute for the Study of War:

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Key Takeaways

• Russia has failed to encircle and isolate Kyiv with the combination of mechanized and airborne attacks as it had clearly planned to do. Russian forces are now engaging in more straightforward mechanized drives into the capital along a narrow front along the west bank of the Dnipro River and toward Kyiv from a broad front to the northeast.
• Russian forces have temporarily abandoned failed efforts to seize Chernihiv and Kharkiv to the northeast and east of Kyiv and are bypassing those cities to continue their drive on Kyiv.  Russian attacks against both cities appear to have been poorly designed and executed and to have encountered more determined and effective Ukrainian resistance than they expected.
• Russian movements in eastern Ukraine remain primarily focused on pinning the large concentration of Ukrainian conventional forces arrayed along the former line of contact in the east, likely to prevent them from interfering with Russian drives on Kyiv and to facilitate their encirclement and destruction.
• Russian forces coming north from Crimea halted their drive westward toward Odesa, and Ukrainian forces have retaken the critical city of Kherson. Some Russian troops remain west of the Dnipro River and are advancing on Mikolayiv, but the main axes of advance have shifted to the north and east toward Zaporizhie and Mariupol respectively.
• Russian forces have taken the critical city of Berdyansk from the west, threatening to encircle Mariupol even as Russian forces based in occupied Donbas attack Mariupol from the east, likely to pin defenders in the city as they are encircled.
• Russian successes in southern Ukraine are the most dangerous and threaten to unhinge Ukraine’s successful defenses and rearguard actions to the north and northeast.
• Russian troops are facing growing morale and logistics issues, predictable consequences of the poor planning, coordination, and execution of attacks along Ukraine’s northern border.

Russia has surprisingly failed to gain air superiority or ground the Ukrainian air force after three days of fighting. The Ukrainian General Staff reported at 11am local time that Ukrainian aircraft conducted 34 sorties in the past 24 hours.[1] Ukrainian air defenses remained active as of 6am local time, with the Ukrainian General Staff stating a S-300 downed a Russian Su-25 fighter and an unspecified helicopter over Donbas early on February 26.[2] Ukrainian forces likely shot down a Russian Il-76 transport aircraft over Kyiv the night of February 25.[3] Russian airstrikes concentrated on northern Kyiv, Mariupol, and unspecified Ukrainian airports on February 26.[4] Russian forces continue to refrain from using the full array of air and missile capabilities available to them. Moscow may seek to limit the informational and diplomatic effects of causing heavy Ukrainian civilian casualties, and may also seek to avoid creating rubble and other obstacles to movement into and through Ukrainian cities. Russian forces will likely increase their use of fires in coming days to overcome heavier-than-anticipated Ukrainian resistance, however. 

Russian forces in northeast Ukraine face growing morale and supply issues, likely due to poor planning and ad hoc command structures, as ISW previously forecasted.[5] Several reports emerged on Ukrainian social media February 25-26 of Russian forces lost and running low on fuel in northeastern Ukraine.[6] A Russian riot police reportedly conducted an unsupported attack on eastern Kyiv and suffered heavy casualties late on February 25, likely symptomatic of wider Russian coordination issues.[7] Several US and European intelligence sources reportedly assess the Kremlin has had to adjust plans and commit more capabilities than anticipated at this point in the Russian invasion of Ukraine.[8]
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There is more.

The Russian troops do not appear to be well-disciplined or led.  They are probably poorly trained and expect to win by numerical superiority.  The Ukraine forces appear to have a competent air defense system and leadership.  They do not appear to have a coherent ground force in place and appear to be relying on civilian fighters.  This could casue the Russians trouble in finding and destroying resistance.