The downsides are obvious
Casting Cost: Card Type: Legendary Creature - Human Cleric Power / Toughness: 2 / 2 Card Text: Flying Whenever Kaalia attacks an opponent, you may put an Angel, Demon, or Dragon creature card from your hand onto the battlefield tapped and attacking that opponent
You'll have to decide which creature type to bias towards
This is, primarily, thanks to the Multiverse Legends bonus sheet, and the return of serialized MTG cards
If Leafkin Avenger leaves the battlefield while its last ability is on the stack, use its power as it last existed on the battlefield to determine how much damage it deals
The two numbers in the bottom right hand corner (4/5, for example) help you determine a creature's attacking and blocking strength, respectively