Yeah, but nagi is something you do as you get hit, no? At least thats how I remember it. Use it after taking a fatal blow, be back to perfect health and a minute of invincibility. For anyone that doesn't have unlimited eyes, it would be trading your sight for staying alive.
Izanami is something you would use before you were definitly going to die, so while it seems like a 100% win if you can pull it off (we're still not sure how hard it is to do so), it is possible that you would be going blind for no reason at all.
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