Submodules don't allow for atomic commits that span repos, though. Atomic cross-project commits are the most compelling selling feature for monorepos IMHO.

(A common case for this is changing a function's API, and fixing all downstream consumers, in one tidy, holistic PR).

> Submodules don't allow for atomic commits that span repos, though.

They can, but there's little tooling support to help you do so.

You can make a new commit in one repo, and then in one atomic commit, you can pull that new commit into a parent repo's submodule and fix the users calling into that submodule.

They don't work quite the same way as svn externals, though, in a way that didn't work for us. In particular, they leave no good way to deal with merging in upstream changes in a submodule's code if you've also made some of your own (provisional) edits.