As I understood:

Izanagi is a defensive jutsu

Izanami is an offensive jutsu

When Danzou was hit fatally, he could change that reality and survive a clearly deadly attack. With Izanami, well, as Itachi said, you can actively decide fate. Which sounds hax beyond belief. I imagine bullshit like "Iīm now imagining my enemy to be dead" and then it comes true. Obviously, that cannot be how Izanami works, so there has to be some kind of weakness. Then again ... who knows