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.



