Generally, most things jj does can be done in Git, it's just much more pleasant / seamless / consistent in jj - maybe with the exception of the "Working on Two Things at the Same Time" pattern[0], that one might be really hard to achieve in Git.

> but does it work as a front-end tool over git/other VCS?

citing from the article

> Before we dive in, one last thing you should take note of, is that most people use jj with its Git backend. You can use jj with your existing Git repos and reap its benefits in a way that is completely transparent to others you’re collaborating with. Effectively, you can treat it like a Git frontend.

> I don't get it. What does make rebase hard?

It's hard when you work on a set of stacked PRs and make frequent changes to arbitrary PRs in that stack, because every time you make a change, you have to manually rebase and push all of the other PRs. There's SaaS's specifically built and used for solving this problem in Git[1].

Maybe hard is the wrong word, it's just annoying and tedious. jj makes this completely seamless, as shown in the article[2].

[0]: https://kubamartin.com/posts/introduction-to-the-jujutsu-vcs...

[1]: https://graphite.dev/

[2]: https://kubamartin.com/posts/introduction-to-the-jujutsu-vcs...