In the case of the card Merrow Commerce, at the beginning of your end step, you can untap all Merfolk you control, which is a huge defensive advantage
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Unblocked Ball Lightnings or direct damage from Vexing Devil doubles up their output, wrapping up games before they even start
Additionally, they are only super effective against a few types Grass, Psychic, Dark, and Fairy and in most scenarios, they end up being outclassed by other creatures with better type combinations
Whats the condition of your cards
You could also change the activation cost of an ability