Can you use Not So Fast to counter Thousand-Tailed Watcher's ability?

Ruling: No, you cannot use Not So Fast to counter Thousand-Tailed Watcher's ability because the ability does not "choose" or target specific units. Not So Fast requires the enemy spell or ability to choose a friendly unit or gear, but Thousand-Tailed Watcher applies a blanket effect to all enemy units simultaneously without choosing them.

Nuances:

• You cannot counter the Watcher unit itself being played, as units resolve immediately and do not stay on the chain
• While you cannot use Not So Fast, you can still react to the Watcher's "When you play me" trigger with other reaction-speed cards before the -3 Might penalty resolves
• This ruling is consistent with similar global effect abilities like Brynhir Thundersong