The Fact she can be summoned 3 times, scale insane high, has synergy and endless rain production combined with the rain decks make her just stupid OP IF you dont run Heatwave, which every Devotion deck does.
Yes she can be counterd with Graveyard hate aswell which, oh well, fucks over Devotion decks aswell.
I cant make up a more unhealthy card for the game which actually FORCES people to run certain cards and play non devotion decks, striping away the fun of going devotion and really into a certain archtype, is just the worst thing I can image for this game.
I would rather play against Mill or Kolgrim ( please dont actually make me face Kolgrim ) all day instead of this rain melusine bullshit
And yes I damn well know its not meta defining but its still a super strong card and the win list of the leader / deck shows this every season.
Source: https://www.reddit.com/r/gwent/comments/tzvbft/melusine_is_fundamently_broken_and_unhealthy_for/