Prismatic Ending is definitely one of the most impactful removal spells Legacy has had in a very long time, and it's extreme flexibility is what makes it so powerful. As with most spells that have X tacked onto them, you can do some cool things with this.
One of the most common ways of using this to interact is to get rid of things like Chalice of the Void. Because Ending specifies mana value or less, it doesn't matter in getting rid of a Chalice what you make X, just that X doesn't equal zero. It also doesn't matter if you have other colors to put into an Ending, so long as you have one color (white) it will get rid of the Chalice.
Tax effects also interact with Ending in ways that are similar to other effects like Engineered Explosives by allowing you to overpay with a similar color or a different color to snag something different but still being able to make X a smaller number. For example, you can hit a Flickerwisp for X=1 with a Thalia, Guardian of Thraben in play by paying white, blue, and green as your color for the additional tax for Thalia. This simply increases the power and flexibility of this card in a lot of ways.