Harnessed Dragon can't be countered with Not So Fast.

Actually, Harnessed Dragon's "When you play me" ability can be countered by Not So Fast.

According to RiftJudge FAQ (ID 11198 and ID 9377):

• Interaction Mechanics: Harnessed Dragon's "When you play me" effect is a triggered ability that enters the chain. Because it chooses an enemy unit (which is a friendly unit from the perspective of the player being targeted), it satisfies the targeting requirement for Not So Fast.
• Official Precedence: While there is a general rule that there is no broad "Counter Unit Ability" mechanic, Not So Fast is a specific, conditional card that governs this exact interaction.

Sequence of Interaction

1. Play Phase: The opponent plays Harnessed Dragon. The unit enters the battlefield.
2. Triggered Ability: Harnessed Dragon's "When you play me" ability is placed on the chain, choosing your unit as a target.
3. State Change: The game state becomes CLOSED (a chain exists).
4. Reaction: You play Not So Fast as a [REACTION], targeting the triggered ability.
5. Resolution (LIFO):
   • Not So Fast resolves first. It counters the Harnessed Dragon ability (Rule 425.1).
   • The countered ability is cleared from the chain and does not execute (Rule 425.1.a). Your unit is not killed.