Double cross was once considered NG's worst leader ability due to the RNG factor and the fact that assimilate wasn't competitive at all. Ever since master mirror was released and imperial formation nerfed, it rose to the top being quite possibly the best NG leader.
It allows you to play 2 or even more cards in the same turn, play the best cards from your opponents hand, forces your opponents into awkward plays to prevent you from getting good cards thus exposing them to poisons/yoinks/coup and so on.
All this while also fitting your entire deckbuilder into the deck.
Not to mention that it can also win you a round that you shouldn't have by skillfully highrolling a great card out of a 6-7 cards hand.
For comparison stockpile and pincer maneuver (two of the least played/lowest win rate leaders) give you respectively 17 and 16 extra provisions.