Wappla hit on something important.
Proactive discard effects like thoughtseize and duress are tempo negative. Don't take it from me either, Reid Duke wrote an entire article on thoughtseize and said "strictly speaking, casting thoughtseize puts you a little behind". This is because you've spent Mana and a card to stay at parity (or more accurately the same amount of card advantage/disadvantage).
This is why thoughtseize was often bad in aggro decks but great in midrange or control decks. It's often times worth the slight disadvantage in terms of Mana spent to strip an important card away, but when you're relying on tempo and speed its often not so great.
The only deck I could think of that would really want an unmask would be storm, but the loss of a card would be terribly damaging to that deck's strategy. In dredge you're relying on cards you can cast with no Mana, especially ones you can cast from your graveyard. Unmask makes much more sense in that deck.
I should mention that there was a fringe deck in legacy called burning Reanimator that used to play unmask. The deck wanted to.set up broken plays on turn one and utilized lions eye diamond, so unmask was a good fit.
If you had a deck that was similar and good enough for vintage, maybe unmask would be OK.