There are a lot of very valid criticisms for Mercenaries, but the one that absolutely baffles me is all the people expecting better visuals.
It's an extra mode using existing Hearthstone assets. The design philosophy is far more "hey, what else can we make this framework do?" than "let's make a totally new game".
Most of that comes from the gameboard being little better than a photographed piece of cardboard. 100% they could have made a more visually interesting field to play the game on. But yes, they treated their new game mode as a cheap asset flip.