I assume this would not stack.
To tone down the power, maybe do one or more of:
Limit the buff to a duration, like how Sundial does not last forever
Add an upkeep cost, like Flooding
Make it a shield card and increase its cost (sacrifice defense for offensive power)
Also, I think the card text should say "All airborne creatures gain _____," since airborne is the word used on Shard of Freedom.