I think the effect is a bit too narrow to be played in an unknown metagame, but in a metagame as you describe, if your list is already running a basic plains it's a reasonable choice, saving kataki and buying a few turns vs oath (if it has angels instead of the simic fattie) is never a bad thing.
The biggest problem i see is that UW fish decks typically have only 1 plains, so replacing it with karakas would hurt the mana base as it's not fetchable.