Izanami is only really useful against someone who uses Izanagi because you have to make your opponent repeat an exact movement. It'd most likely be known by Madara because Itachi states that too many people were using Izanagi and Izanami was made to counter it. So it would be safe to assume quite a few of the Uchiha at least knew about it. Also you lose one of your eyes. It's only other effective use has been to immobilise an opponent like Kabuto who could easily overcome it if he accepts his destiny. This is why it's a kinjutsu, overall it's not great. considering the loss of an eye and the skill required to make an opponent repeat the same move.