The truth of the matter is its the fault of the us testers for not considering the full ramifications of a card.
However to be fair, we are human and have faults. Cards that are part of tribes that we're trying to get a little more play for tend to be a bit over looked if a much lower rarity card, especially if it falls into the more support oriented mix. On paper Harpist is a pretty good card for trying to improve the extremely underplayed mermaids. As a result it didn't quite get the level of testing that a card like Rivera or Veerlat got. Partly due to the rarity and the fact that they have some pretty set up niches, that already see a bit of play.
the mermaid issue will probably be fixed soon.
His blood is of iron and his heart of glass
He survived through countless battles
Not even once retreating
Not even once being victorious
The bearer lies here alone
Thus, my life needs no meaning
This body is made out infinite swords