Mercy has been nothing but a controversy ever since people saw how useful and impactful she is and can be. All the other healers are literally a downgrade because of her infinite healing and the res cooldown.
In a game like this, infinite healing and Resurrection will always take priority over anything else. Her design is flawed for a game like this. She is and will be the best healer unless you completely rework this character like Symmetra and change all aspects, or just break her down completely and make her useless.