"We can be certain that Putin will mount a counteroffensive to try to reclaim that territory," Cohen said. "I think our expectation is that that will be a difficult fight for the Russians."
Putin, he said, "is not only going to have to face the fact that there is a front line now within Russian territory that he's going to have to deal with, he has to deal with reverberations back in his own society that they have lost a piece of Russian territory."
Remember, the area that Russia hit is area that Russia intends to annex; i.e. they want it to be part of Russia. If they're willing to hit that, I'm not at all sure that hitting area in Russia is off-limits.
Also, currently the area that Ukraine holds in Russia isn't terribly populated. I haven't been watching closely, but I think the largest was Sudzha.
It has a population of 5,127 people.
That's probably at least partly evacuated and not exactly a metropolis.
Russia is 100% about optics, what things look like is more important than how they actually are.
The optics for a) losing territory and b) bombing your own territory is not good. A good Tsar never loses ground, a good Tsar increases the size of Russia