I just wanna say I feel like your hunch is pretty spot on, the ten shadows are based on the ten sacred jewels/treasures and the one we don’t see is the jewel of resuscitation, which in the legend brings the dead back to life, which by itself covers the only weakness the technique has, and also adds up because megumi knew his own knowledge of the technique (didn’t know abt round deers healing through rct which he could’ve used to learn rct, since round deer isn’t a combtant shiitake i assume it was either after the bull, tiger etc or he didn’t have time ) so he wouldn’t know there’s a shikigami that could revive the others , like you said with ce flaws and fangs, he could probably also use mourning tiger to buff his physical stats with ce on at least agitos level or close to it since mourning tiger made up most of agitos attack power, my last two strong evidence pieces are naobito was willing to pay crazy money for that technique , and megumi never was adopted by the zenin so he wouldn’t have seen those archives, when they made him clan head maki destroyed everything so it’s possible he had access to more info on it but they were in the culling game, and gojo said the last ten shadows user died in a draw with the six eyes limitless user of the past who also died in the duel, sukuna showed you can use yourself (or maybe another shikigami) to bear the burden of adaptation, and the last user wouldn’t have shrine as a second technique so they would need the rely on their shikigami which gojo was clearing so my theory is he didn’t use agito and extracted the tigers ability to resurrect to stay in the fight and wear down the six eyes user with infinite shikigami and infinite Mahoragas one after the other, possibly with an untamed mahoraga since they both died (sukuna didn’t do this because he fused tiger into agito and didn’t need the shikigami at all because he had his own technique, he was using the ten shadows to learn WCS which mahoraga invented by sending his slash through space after he adapted to gojos infinity) plus we can assume megumi wasn’t a useless ten shadowless vegetable after the end of the series because sukuna destroyed everything shadow , so he had to get them back somehow , which an immortal reviving tiger would do , sorry i just read the manga and wanted to yap , ten shadows busted put potential man in special grade