There are a few key differences, like that split cards have the combined mana value and types of both halves, while MDFCs only have the properties of the front half when anywhere other than the stack or the battlefield
For me, this dragon is just slightly outclassed by Counterspell in our ranking
Sandsteppe War Riders works particularly well with Schema Thief since you can make copies of artifacts that arent even in your deck when it deals damage
There are just so many cards that work well with Birgi that you can capitalize with something as simple as Burning Prophet and Guttersnipe, or as complex as Defiler of Instinct
Payoffs like Cauldron like good enablers, things like Steel Overseer are not good enough
Thats why this decks so good at mana ramping, deploying large creatures, and using strong spells and abilities to increase your power in the arena