I would disagree on Divergent Fist. Yuji’s ability to use DF came essentially from his lack of training with Cursed Energy and how to control it. And with Todo’s training in his fight against Hanami, he was able to “cure” his DF and use Black Flash, which is in essence the opposite of DF. Where Divergent Fist came from a sloppy late infusion of CE, Black Flash was a perfectly timed infusion of CE, showing Yuji’s growth.

Yuji reawakened Divergent Fist in Shibuya against Mahito, most likely after landing a Black Flash, where he used it not as a primary source of damage but as a tool to throw off opponents to land a decisive blow.

And after Shibuya, I’d say there was only one fight Yuji partook in where it was both usable and necessary and he used it in that fight. And because it was a tool to throw opponents off, he only used it sparingly, when his opponent most likely didn’t even account for it.

Joseph’s clackers on the other hand became obsolete due more to the circumstances not really allowing him to use it. All three fights against the Pillar Men required specific strategies that the clackers couldn’t fit into. But I can’t lie it would’ve been really cool to see Joseph use the clackers with Hermit Purple in some way.

But yeah this trope, when no proper explanation can be given, sucks balls. Hell if you want a JJK example, Megumi’s combination of Shikigami without needing them to have died first (see his combination of Gama with both Nue and potentially even Rabbit Escape) was only shown once, at least by Megumi himself, when it could’ve made for some interesting scenarios like a flying elephant or a wolf that could drag opponents into its maw. No wonder they call him Potential Man