If an effect prevents playing "action" cards, can Promising Future's effect still allow playing any card type, or only reactions and units?

Ruling: When a spell or effect instructs you to play a card, you can play any card regardless of timing restrictions or other limitations. Timing restrictions only apply when playing cards as discretionary actions on your own.

Nuances:

• The distinction is between playing cards when instructed by a spell/effect versus playing cards as discretionary actions
• Promising Future instructs you to play a card, so normal timing restrictions do not apply