I tend to have lots of uncommitted files and changes that i want to keep around in this state while I move around branches and while having multiple change lists (jetbrains implementation) that I will commit at some point in time.
This loose, flexible way of using git seems hard to do in jj.
It's also so easy to go back to the change latter and remove the files (after they're already copied elsewhere, or just operations log to go get) that it's really not a problem to just let stuff get in your commits.
In git there's such a strong incentive to do things right, to make clean commits. Imo one of the huge strengths of JJ is abandoning the obsession, and having far far far better tools to clean up after.
There is no such. There are a lot of tools to manipulate commits and WIP, such as the stash, rebase, cherry pick, extracting and applying patch. You only need clean commits for review and the main branch because that helps the whole team. Your local copy can be as messy as you want to.
Where-as jj makes rework operations, IMO, basically easy enough (after a couple weeks of use to) that I worry much less about making the right commit the first time.