I've done extensive testing of regular units, and it seems that Hydras consistently beat Niefel Jarls. In fact, I haven't seen a unit that beats the Hydra from the regular buildable units.
Aren't chill effect and Breath of winter two separate things/effects? Chill will cause fatigue and freezing, while BoW will actually cause damage like a cloud spell, if I understand correctly. That would mean that it's worth to cast it with your Jarls to make them even more deadly.