but it is good against merfolk and goblins.
Are you saying this because you actually tested it against Merfolk and saw that it was good? Or did you just read the card, decide that you think it would be good, and then tell us that it was be good by fiat?

Plague is bad against Merfolk. Merfolk runs about 12 cards that cost the same as Plague, counteract Plague, and can arrive free of charge via Vial.