In 2010 North Korea shelled a South Korean island and about a quarter of the shells turned out to be duds. A lot of the rest proved to be extremly inaccurate. That is not to say they are not a danger, but they are most certainly much worse then new NATO shells. That is not even talking about range.

The artillery situation already turned from Russian dominance to it being fairly balanced with Ukraine having a slight supremecy in some regions. It looks like highly accurate systems like PZH-2000 combined with artillery radar are slowly but surely taking out so much Russian artillery that they are depleting their stockpiles.

As for manpower Russia has basicly a three to one advantage, but Ukranians seem to be more willing to fight and Russia has to pull them from the weakest in society. Otherwise Putin is threatend by a coup.

Russia will start to have really big problems as soon as the oil price falls.