I mean Valve once said that QA is the most vital part of game development.

Their classic anecdote is about Portal's Development. They brought in some players to play the game near its completion and the user response was "Nice Demo, can't wait to play the full game." And they decided they had shown the player how to implement basic gameplay mechanics well, but that they hadn't given the player a satisfying enough situation to use all of the mechanics at once. This is why Portal ends with the most amazing bossfight Valve has ever made imo.