Can hidden cards be played in response to units being played if those units don't have WYPM (When You Play Me) effects?

Ruling: Hidden cards follow the same rules as regular reaction speed cards. They cannot be played in response to permanents without a trigger (such as units without WYPM effects).

Nuances:

• Hidden cards literally gain the reaction keyword
• They must wait until the next turn after being set to become usable
• They get appended with the battlefield they are on for targeting purposes
• They can respond to units with WYPM effects because those create triggers on the chain