I suppose it depends what you mean by "horribly break things".

The only thing I've noticed is that `jj` will leave the git repo with either a detached HEAD, or with a funny `@` ref checked out.

I don't think that would trouble someone who's experienced with git and knows its "DAG of commits" model.

For someone who's less experienced, or only uses git for a set of branches with mostly linear history (like a sort of "fancy undo"), I could imagine getting a shock when trying to `git commit` and not seeing them on any of the branches!