Activating a creature card’s unearth ability isn’t the same as casting the creature spell. The unearth ability is put on the stack, but the creature card is not. Spells and abilities that interact with activated abilities (such as Stifle) will interact with unearth, but spells and abilities that interact with spells (such as Cancel) will not.

If a creature returned to the battlefield by the unearth ability would leave it for any reason, it’s exiled instead—unless the spell or ability that’s causing the creature to leave the battlefield is actually trying to exile it. In that case, the spell or ability succeeds at exiling the creature. If the spell or ability later returns the creature card to the battlefield (as Astral Drift might, for example), the creature card will return as a new object with no relation to its previous existence. The unearth effect will no longer apply to it.

Dregscape Sliver grants the unearth ability to other Sliver cards only while it’s on the battlefield.