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Considering Slither Wing only has 140 HP, leaving it with only 30 HP on the field is a big problem

Beyond that there is how immediate the life gain is, how much you get or can potentially get, how cheap the card or effect is and how hard it is to disrupt

Set 1 Fire Formation Spell/Trap directly from your Deck or GY

This card, Leyline of Lifeforce, would greatly benefit a deck focused on playing powerful creature spells that are vulnerable to being countered

Benthic Biomancer Another merfolk all-star, Benthic Biomancer goes in more decks, including counter shenanigan decks like Chisei, Heart of Oceans or Goldberry, River-Daughter