There's talk about iffy Russian morale. A large %, up to half(?), of the Russian invasion force are said to be conscripts on 12 month stints. Reportedly many were told they were going on a training exercise when sent to the border areas. Presumably they were intended to be used as mopping up and hold forces after quick Russian victories. If such inexperienced soldiers are forced to fight, that levels things some.
Some of Putin's rhetoric works against him -- if the Ukrainians and Russians are one people and brothers, why kill them? Obviously protecting the Donbass Russian speakers doesn't require a mass invasion of the country and capitol. Just insert Russian troops into those two provinces and any perceived threat is over. Have to wonder how much Russians believe Putin's rhetoric.
Putin reportedly sent in ruthless, battle-hardened Chechen units and thugs. They might know urban combat, but also in Grozny they just leveled much of the city to root out resisters. Intentionally targeting hospitals or apartment buildings is a favorite Russian tactic, used in Syria and Chechnya, designed to instill panic and lower resistance, make it seem Russia has more missiles than it does, and of course will be lied off as accidental.
Kazakhstan declined sending troops after Putin's request.
Maybe if Ukraine holds out a few more days, western missiles and other weaponry might be enough to delay Russia, mess with their supply lines, conduct rearguard attacks on their weakest soldiers, keep an armed resistance going. Some talk about some Russian soldiers being poorly equipped. Dead soldiers coming back to Russia isn't going to be good for Putin.