Can you play a reaction in response to Brynhir Thundersong being played?

Ruling: You cannot play a reaction to Brynhir herself being played (she resolves immediately as a unit), but you can play a reaction to her play effect when it's on the chain.

Sequence:

• Brynhir is played and resolves immediately to the battlefield
• Her play effect goes on the chain
• Reactions can be played in response to the play effect on the chain
• Once the play effect resolves, no more cards can be played that turn

Nuances:

• Units resolve as soon as they're finalized to the chain, so the unit itself cannot be reacted to
• The play effect lingers on the chain separately and can be reacted to