I hear this a lot and I really don't agree with it. There's nothing particularly "challenging" about utilizing cleric stance. Healing requirements in end game content in this game are designed very lazily, typically with very predictable damage patterns and long periods of either completely negligible or solo healable damage. Do a fight one time and you'll know when you can and cannot safely be in cleric every single time after that.
It's an illusion of challenge that simply exists to satisfy SE's penchant for complication.