You can kind of break the whole game with the Phase locket. Shovel Knight is a pretty rough game if you're going for the no relics achievement.
I never really used the Phase Locket except in one case that you probably know about. I feel like maybe Patrick did as he suggested to Alex that he could use that in a bind. And if he did use it more often, the game could definitely be made a lot easier. It's not that it doesn't make sense and I probably should have used it here and there, but I just forgot about it. I mostly swapped between 2 other relics other than the healing items.