Would a Unique Treasure ability that prevented that treasure from being removed from the game be too powerful?
I’m trying to think of a way for a face-up UT to not be removed by other Treasure’s such as Pirata Codex and Kharmic Idol.
That would depend entirely on what else the UT did.
The Codex, Idol, and Jailhouse Dog exist mostly to counteract the crazy powerful UTs that can be otherwise hard to get rid of, so anything specifically designed to be immune to them would have to be considered very carefully.
Mechanically, all it would take is a line saying something like, “This unique treasure cannot be removed or eliminated by other unique treasures.”