I wouldn't say 'game-breaking', but I might say 'game-changing'. There are certain kinds of challenges that just become meaningless with this kind of thing.
Pretty much this.

We have a DMM cleric in our party, and with lesser vigor persisted, we really ignore things like one of us falling at negative HPs, the wounds we suffered during a battle, or the chest that might be guarded by a burning hands spell that is activated if you try to crack it open instead of finding the key.