Can a player use reaction spells in response to an opponent playing a unit from hand before it hits the board?

Ruling: No, you cannot react to an opponent playing a unit. When playing a permanent (including units), priority does not pass on the chain, so there is no response window.

Nuances:

• The rule specifically applies when the card that INITIATED the chain is a permanent
• If a permanent is played in response to something else on the chain (not initiating the chain), it may be possible to react to that permanent