The set, which is over a year old now, was the fourth released, and contains 204 cards
The fact that split cards change whether its in your hand, graveyard, or on the stack is important because of how rules consider each spell
(Though Ozai, the Phoenix King does this even better, allowing you to keep the excess mana.) Miirym, Sentinel Wyrm is solid, but only in green-blue-red dragon decks and nowhere else, so that's a tad limiting
That said, they have some drawbacks you should be aware of: Some weak cards : This is, in part, a side effect of the fact that precons are designed to starting points which players build on: there are some weak cards which are meant to be identified as underperformers and subsequently upgraded