If Tornado Warrior is revealed as a hidden card in response to an Unchecked Power, does Tornado Warrior still get hit by the Unchecked Power or is he the only survivor on his battlefield?

No — Tornado Warrior takes the full 12 damage like every other unit and dies with them.

Unchecked Power deals damage to all units at battlefields — no targeting, no exemption. Hidden units gain Reaction while facedown (Rule 811.6), so you can flip Tornado Warrior while Unchecked Power is on the chain, but the chain resolves LIFO: Tornado Warrior finalizes to the board first, then Unchecked Power resolves and hits him. Being "played in response" doesn't let him dodge the damage — there is no window after Unchecked Power resolves to play him (FAQ 7958).

His play-from-hidden ability only empowers something (a binary status that does nothing on its own) — it grants no damage prevention or toughness. With 3 Might, 12 damage is lethal, so he's trashed in the cleanup along with everyone else. He is not the sole survivor.