What about Zhonya’s Hourglass and Stand United? When they’re played from hidden, are their effects restricted to the battlefield where they were played?

Only effects that choose are restricted. Zhonya’s Hourglass saves the next friendly unit that dies anywhere. Stand United can only give a buff to a unit at the battlefield where it was hidden, but its other effect, which causes buffs to grant an additional bonus, applies everywhere. This is a change from previous rulings on these cards, intended to simplify the question of what hidden cards can and can’t do. Zhonya’s Hourglass has additional rulings and minor errata related to saving dying units; see below.