Rather than izanami or izanagi (like lots of people are saying), I'd argue that six paths sage mode was the hardest thing to understand (I know that is a mode rather than a jutsu, but close enough).
If you think about it, it took us as a fan base forever to understand what exactly RSM is. In fact, every now and then I still see threads debating about RSM and whether or not Naruto still has it. Even after the 4th data book came out, it still took people (myself included) a while to get the memo that the form Naruto donned when he kicked Madara's truth seeking ball was RSM, and that the golden form with 9 truth seeking balls is a higher form of RSM powered by the 9 bijuus.
This confusion was made even worse after chapter 699 when the forever nerfing Kishi decided to not have Naruto use RSM at all in the Last and then introduce this "Kurama chakara only RSM" in Gaiden and the Boruto movie. For the longest, many people including myself thought that this mode was simply BSM in Gaiden. Some people still do believe that in fact, and debates actually still go on about that from time to time to this very day. It doesn't help one bit that Kishi actually gave Kurama chakara RSM a design that looks just like BSM, and he took Naruto's truth seeking balls away from him. At the very least, Kishi could have let the design keep the 9 tomoe and the rinnegan symbol. The only way that we can distinguish that adult Naruto is using RSM is because he lacks sage mode pigmentation around his eyes while he has RSM cross eyes and his jacket is short like that of RSM (as seen in the Boruto movie).
The method to obtain RSM wasn't made very clear to begin with either.
For example, we know that Asura and Naruto got it as a powerup from Hagoromo. Hagoromo was presumably born with this power. Madara and Obito got it due to becoming the 10 tails jinchuuriki. However, confusion comes in when you think about Hamura and Kaguya. Kaguya was the one who ate from the shinju itself and she herself (fused with the shinju) is the juubi. Like RSM users, Kaguya can also produce truth seeking balls and fly. Nevertheless, Kaguya is not viewed by Kishi or the data book as a user of rikudou senjutsu? Similarly, Hamura came from the same mother as Hagoromo, and his chakara (like Hagoromo's) is powerful enough to induce the ability to fly, the creation of truth seeking balls and even Hamura's lineage's own equivalent of the rinnegan (the tenseigan). All of these traits are near identical to the traits of Hagoromo's chakara, yet Hamura is not a user of RSM?
Also, some people are confused about the difference between rikudou chakara and rikudou senjutsu. Many people have speculated that Sasuke has senjutsu due to having rikudou chakara (which people believe to inherently contain senjutsu). Despite this belief (which may or may not be true), Sasuke did not receive RSM just as Naruto did not get rinnegan from his rikudou chakara.
Do you see what I mean? The introduction of various forms of RSM, the design that adult Naruto's RSM takes, the power that adult Naruto demonstrates, the ambiguity about what conditions you need to obtain RSM (whether it is rikudou chakara/senjutsu/bijuu power, or just rikudou chakara/senjutsu, or if rikudou senjutsu is just one thing all on its own and the senjutsu doesn't have to be added to rikudou chakara seperately) , the inconsistency about who gets RSM and who doesn't , and just some general plot points that Kishi could have written better make RSM one of the most confusing abilities in the series.
What do you guys think?
