Russia managed in Chechenya, once they got their marbles together.

It's conventional warfare, not guerilla warfare, nor a suicide bombing campaign. The worst case scenario would resemble the Iran-Iraq war and the initial conditions resembled Northern Cyprus more than Afghanistan.

The Ukrainians don't really even have an equivalent to the Viet Cong. Insurgencies need the support of the local populace to eat and operate without being ratted out. Only Kherson is even close to divided enough for that to have a remote chance of occurring.

The terrain and culture of Ukraine aren't like Afghanistan. Afghans are more loyal to their tribes than Kabul. The underdeveloped subsistence farming economy of Afghanistan means any insurgent knows how to live off the land and survive living in deprivation for long and sustained periods. The terrain of Afghanistan allows insurgents to easily isolate whatever forces the government sends in an attempt to control the countryside. The Taliban won without any state really backing them. The overall population of Ukraine and Afghanistan might be similar but what matters most is the demographic in the age range for military service.

Syria didn't turn into an Afghanistan. The Donbass republics fended for themselves for 8 years while the DRA only lasted 3 years alone. Even then the DRA still outlasted the Soviet Union so had material support continued they might've continued on as a rump state. It wasn't like the US puppet, Ghani's regime, which just collapsed immediately after the US pullout.

Ukrainians are more Northern Alliance than Taliban. Their ideology isn't anywhere near as unified as the Taliban's. The moderate liberals and extremist fascists would turn on each other if living conditions deteriorated sharply.

There can be no defeat either. Pulling out would be political suicide for anyone in the Kremlin. Abandoning Russians in Russian territory would be different from abandoning the DRA. Nukes would fall before Crimea falls.

Anyway, you can't really predict the future by looking at the past. The similarities are only down to hindsight and brute force exhaustion of every possible historical parallel.

The Soviets would have succeeded in Afghanistan if it weren’t for Gorbachev, besides western equipment is running out and more apathy is growing in American and NATO brass.