If I use a spell that chooses multiple minions, if my opponent uses Not So Fast, does it cancel the whole effect of the spell, or only the application on the unit they used Not So Fast on?

If you play a spell that targets multiple units and your opponent plays Not So Fast, it counters the entire spell, not just the effect on one unit.

According to Riftbound FAQ #8423 and Riftbound FAQ #7323, when you use Not So Fast to counter a spell or ability that chooses a friendly unit or gear, the entire effect is cancelled. Even if the spell has multiple targets or uses the "Repeat" mechanic to affect multiple things, Not So Fast counters the spell as a single chain item.

Why this happens:

• The Chain is Atomic: When a spell is placed on the chain, it is treated as a single object.
• Countering the Spell: Not So Fast targets the spell itself. Because the spell is a single chain item, countering it prevents it from resolving entirely, regardless of how many units it was intended to affect.
• No Partial Resolution: A spell does not partially resolve just because one of its targets was "countered" or invalidated by a counterspell.

In summary, playing Not So Fast completely cancels the spell, and none of its instructions will execute.