These might be specific to the permanent type you sacrificeDisciple of the Vault for artifacts, Hearthhull, the Worldseed for lands, etc., or more general payoffs like Mayhem Devil and Evendo Brushrazer
So that's 8 cards you should definitely keep
Phasing is better than giving hexproof instantly (Lazotep Plating, Heroic Intervention) or indestructible (Rootborn Defenses, also Heroic Intervention) - which are heavily played
Previously, in cases where a card's mana value differed from the resulting spell, such as with some modal double-faced cards or cards with an Adventure, you could cast a spell with a higher mana value than the exiled card.
Frenzy This mechanic was made as a mechanic to encourage blocking
In the end, its just a 2 mana Firebolt