You're both working on the same branch on the remote? It will refuse to push unless you do `jj git push --force` at which point it will overwrite your coworkers work with yours. This is regardless of conflicts, like with git you have to pull the remote work locally, rebase on it, and then push it.

> iiuc jj uses stacked commits instead of branches

Locally jj doesn't really use branches, you can just create a commit on top of any other commit without creating a branch. It does represent branches (it calls them "bookmarks" for whatever reason) and pushing is pushing a bookmark to a git-branch on the remote.

> Are my builds and PRs now at the mercy of changes and events outside of my control?

I don't believe there's any circumstance where your work is silently rebased on top of other work, or anything else that fits this description.