The idiocy that is "staging" is the primary problem with Git. 99% of people do not need "staging" and yet Git inflicts it on everybody.
"Staging" (and "stash") makes Git modal, and that's a pain in the ass.
Having taught people both Mercurial and Git, I can tell you that teaching Git is simply a losing battle. Fortunately, jj operates just like Mercurial but can coexist in the Github ecosystem.
From what I can tell about JJ's split command, it seems like it does the same thing except you're working backwards to split out changes into another commit.
Both seems fine, I just don't see the complexity with the staging area (unless you're a beginner who's been told to only use the CLI at all costs).
Jujutsu's authors have pointed out that a simpler model (here, and with first-class merge conflicts) helps them move much quicker. There are many weird and exceptional states with Git that just don't occur, so new features are more straightforward to add.
Shame Git won so vehemently, most software projects don't need that much power (and guns to shoot their own feet), but cargo-culting after Linus was too strong a lure I presume.