Board Thread:Theories and Speculation/@comment-35685868-20180605140514/@comment-26107879-20180701054106

"reputed for having advanced knowledge and/or skill of all the techniques to ever exist within Konoha, including the many Hiden techniques." Chapter 120 and 4th Databook.

Hiruzen pretty much knew every jutsu in Konoha. People seem to forget how old Hiruzen was. He was 70 years old. His Chakra was so low at that point, that it was stated that even making 2 Shadow clones was a risk for him. That's why he never showcased such deadly abilities. You expect him to be using Flying Raijin alive? It literally takes 4 Jonin to do that. Hiruzen didn't have the reserves to do it at his old age. His old age cursed him with low reserves.

When it came to the 4th War, he wasn't as inhibited. He could use up all his Chakra just to have it replenished. It's possible he might have been able to use it. But he still had low reserves, they just would fill up eventually after usage and wouldn't kill him if he ran out. So I don't know if he even had enough initial chakra to make a single jump with Flying Raijin. All Rijin requires is knowledge and Chakra. It's obvious Hiruzen had the first.

Then the situation of the 4th War. It's not like they were summoned and could go grab everything they needed to fight. No. They were summoned and the moment they could, they left for the war. Hiruzen did not have Flying Raijin Kunai's with him, so he wasn't going to use it in war even if he could.