2nd page is only a problem with Young Witch and Looters, non supply piles are ok on another page imo.
To build an interface that specifically only fits an exact number of cards at once, while they knew ahead of time that this wouldn't work for all expansions, is a problem for me.
This.
To elaborate, the reason this is bad UI design is because most of the time—when you don't have a YW or Looters in the Kingdom—you don't need to change pages to see if the game is about to pile out. As a result, the user is trained to not think about the second page. Now, when some of those cards do show up, the UI's rules change: relevant info for the endgame state is split over both pages. It's that difference that's the problem.
This is arguable, but I believe it would be better for the game to need a second page ALL the time, then to need it some of the time. The tradeoff is between "should info on game state be consistent" (then put it on two screens always so the user is trained to flip between screens to check all the cards) and "fewer clicks to see the relevant information" (do what Goko does currently). Regardless of which you think is better, modifying the layout so Ruins/Bane are on the first page, even if Prizes/Spoils/etc are not, would accomplish both goals better than either halfway solution.