In colocated repos, running both git and jj commands is supported. I use it in release workflows which require git tags, which jj does not support creating. However, jj will pick up ("import") on git-created tags just fine afterwards.

AFAIK, jj runs "import" before and "export" (to git) after every invocation. That means it always has a consistent view.

jj can also handle concurrent edits by itself, think in a repo shared across a network. That said, I wouldn't think concurrent git commands are safe.