McMaster is making some of the same points that I've heard from other generals with invasion and occupation experience, like Petraeus and McCaffrey:
Putin's army just does not have the capabilities and the troops to pull off what he planned. So they're increasingly just substituting explosives.
Just look at simple numbers:
The active-duty strength of Russia's army is given as roundabout 900,000. That's not a lot of soldiers for a major land war. During the Cold War, West Germany alone had 500,000 on active duty, and that was for a 100% defensive posture. In contrast, Putin wants to overrun and control a sizable country where everybody resists.
And urban combat can be brutal, as long as the defenders are willing to die. Blitzkrieg-intoxicated historians have tended to skip over this, but when the Wehrmacht rolled into Poland in 1939, it took them eight days to reach the outskirts of Warsaw; then they got stuck for three whole weeks in the suburbs because the blitzkriegers turned out to be lousy urban warriors, and the Poles fought.
Considering how close Kyiv and Kharkov are to the Russian border, I think we're witnessing a re-run of that scenario. Except that in contrast to the Nazis, whose Soviet partners-in-crime attacked and sealed Poland's fate, Putin has nobody to help his army get unstuck.