Ukraine Insurgency vs Russian Invasion

What makes urban warfare/insurgency in Ukraine (against Russia) different from an authoritarian government invading/controlling a US state? I don’t hear nearly any jokes about how it’ll be the next Vietnam for Putin invading a hostile country. When there’s wild rumors of the US invading Texas/Conservative state, we get all the bravado about how well armed the average local is, how bad the losses will be, and how terrible the PR is. Isn’t the average Ukrainian almost as well armed, and trained as the average conservative citizen? Won’t they be just as motivated to kick an occupying force out in any way they can?

Is there an assumption of compassion/ruthlessness in the Russia/US comparison(in regards to their anti-insurgency tactics)? Why isn’t Putin nearly as afraid to fight urban/insurgent warfare in Ukraine? Is it the terrain, small size, & density?

*I’m assuming Ukraine loses the war and nobody else supplies troops, but still has rebellion.