One of the main reasons to use git-branchless over jj or Sapling is if you need support for a workflow that's not supported in jj/Sapling yet, such as Git LFS. I (git-branchless author) would like to see both systems get to feature parity one day to unblock users who would like to migrate but can't.

These days, there are relatively few things that git-branchless can do that jj/Sapling can't, one example being its (somewhat opinionated) `git test` command: https://github.com/arxanas/git-branchless/wiki/Command:-git-...