Can a hidden unit be played/flipped if its ability has no valid target, or does it become unplayable?

Ruling: Hidden units can still be flipped and played even if their abilities have no valid target. The unit enters play, but the ability simply does not resolve.

Nuances:

• If a unit's ability requires targeting and no valid target exists, the ability does not happen but the unit still enters play
• Units with abilities that can target themselves (like Blastcone Fae) must do so if they are the only valid target
• Blue Teemo has a special case under review: it's unclear whether you can reveal the top 5 cards without having a target for the damage portion of the ability