Board Thread:Theories and Speculation/@comment-3001903-20160813043039/@comment-25122992-20160820124858

The problem is that all the known WR users except for Hashi (and anime-only Asura) got their WR through his DNA, so when their use was called "nothing like Hashi's", it was meant that they are just weak copies of Hashi with nowhere of his original strength. Also, WR was one of the most popular abilities to be experimented on, which is explained by both its exceptional stregth (particularly, the ability to control the Tailed Beast) and rarity (since there's still no records about its users aside from Hashi and his experimental "copies").

As far as I know, almost all of the non-Uchiha who witnessed Izanagi didn't make it to survive. And Uchiha themselves labelled it a forbidden jutsu which was kept in secret. But you can't be sure that Izanagi was completely unknown to the shinobi world. For example, Danzo (and presumably Orochimaru) found out about the technique and even learned to perform it and extend its use, again via Hashi's cells. Regarging Susanoo, I wouldn't say it was such an obscure power, since quite a lot of people (the primary examples being Danzo and Kakashi) managed to recognise it on sight, although this technique was actually considered to be very rare to awaken, even amongst MS users (as noted by Obito).