Relative to each other. Unless I’m misunderstanding something, Unholy’s set bonus is just objectively better than Frost’s, to the point where you can argue that Frost’s is redundant.
Unholy has capped Winning Streak which lasts as long as Dark Transformation and cannot be removed for as long as DT is up. Frost has increased stacks of Winning Streak and free Frostscythes, plus Frostscythe benefits from Winning Streak with the 4p bonus.
So, slightly lower buff ceiling that is guaranteed to last 30 seconds, versus slightly higher buff ceiling that can instantly expire and has to be re-applied, and buffs to Frostscythe.
Obviously we won’t know for sure until PTR, but at least on paper, Frost’s tier bonus is dogwater, Unholy’s clears it 7 days a week.