Here is a game where this messed up and even banned a card that Warchest not only would never want, but can't even gain as BV costs 6. Later in this same game I was sure he was playing a Bishop and so i preset the curse for trashing thinking that even if he doesnt trash i can just play my actins as the preplay wont preplay a coin or curse on its own if not in the buy phase. .... and then War chest banned curse
At a BARE minimum it should only ban gainable cards like between 2 and 5 b/c unless an EXTREME EXTREME edge case no one is EVER going to warchest gain a curse. I guess in Tower or a 3-pile situation but in those edge cases not using preplay would be the better option.