Winter in Ukraine can be lovely. Kiev becomes a snow-covered winter wonderland with the river Dnieper frozen into an ice-skating rink. Cold is OK if you wrap up warm.
The only snag over christmans / new year is the large number of pulblic holiday: western xmas then western new year then orthodox xmas then orthodox new year. Ukrainians tend to spend all holidays with their families rather than going out so everything can seem a bit quiet with erratic opening times for shops / museums etc.