The highlighted.The Russians know how to deal with insurgents in an urban environment far from their international borders. It's probably a bit different when you're creating a humanitarian crisis and committing war crimes just across the border from mobilized NATO member Poland.
And don't forget that NATO has some experience dealing with separatists, nationalists, and war criminals in Eastern Europe. And some experience dealing with aggressive dictators fielding poorly trained troops and obsolete Russian military hardware.
I think this conflict will take on a very different tone if Putin starts doing things to Ukranians that the rest of the world simply cannot ignore.
And the Russian people.
Information is more available to Russians citizens than in the Soviet era. And videos of atrocities or bodybags, and defiant last stands by people who are supposed to welcome Russia is not going to play well with them.
Whilst the potential cost to the richest is almost certainly getting them to sharpen the knives (very quietly and very much in secret)