Not sure how this deck shows the nymph buff is OP, I've been getting 6-7 turn wins with it, which isn't unusually fast, and it doesn't do much else that other decks can't do.
Imo a card is OP if:
1. It makes another similar card close to useless.
or 2. It makes a new grinder possible which is much better than any existing grinders.
Until someone creates a great new FG deck using nymphs with a 75% winrate, I'm not convinced. Pretty awesome deck though