A sharingan's Izanagi was only used to escape death, just as how the Izanagi god escaped the yomi underworld, running away from Izanami. The sharingan's Izanagi is only a mere aspect of Izanagi's power. Izanagi's true power is the Rinnegan Izanagi, controlling all creation. And since the eternal Mangekyou is on a higher level than the sharingan, it only goes without question, that the Mangekyou Izanami would be far more greater than the sharingan Izanagi. It would still be below the level of Rinnegan Izanagi or Rinnegan Izanami but enough that we can call it the most deadliest of all jutsus.
It's possible that Uchiha Madara himself never used it, cause he had other plans, meaning it would use up a whole lotta lifeforce, he didn't want that, he instead wanted to become the full Rikudou, so he could use the maximum power of the rinnegan. However Sasuke will have no such hesitation to use Izanami.
I'm betting it'll be Sasuke's 4th Mangekyou (EMS) jutsu. And I'm betting it'll be an onmyoton release as well, cause Izanami was the creator goddess alongside Izanagi before she became death and destruction.
Onmyoton release uses both yin and yang, so it'll most likely take a huge chakra toll on the Mangekyou wielder, kinda used as a last resort. A sharingan wielder could use Izanagi, but at the loss of the eye, and huge chakra dip (cause they have low yang reserves), Danzou compensated that by using Shodai's energies for ten Izanagis.
The rinnegan wielder uses a perfect Izanagi, countless times, Rikudou used it least 9 times for the bijuus, and another one possibly for Gedou Mazou, who knows what else.
So the rinnegan wielder would use the perfect Izanami as well, the Mangekyou EMS Izanami would be imperfect but still very effective, as it'll be the most deadly and dangerous jutsu ever shown in the manga.
Btw, it's possible that Izanami after dying and becoming death and destruction, would've lost her ability to create and give life. So whereas the Rinnegan Izanami would be an onmyoton release, the Mangekyou Izanami would only be an inton release, meaning the Mangekyou wielder can use it more than once without huge chakra losses. That depends, can't say for sure right now, will need more info. But I know Izanami was originally a creator goddess, so that would've put her on as an onmyoton release, using both yin and yang, but once she became only death, then she would be an inton or yin release. This would suit her better as in the Mangekyou are the death powers of the spiritual realm, the underworld, where Izanami resides.