Anything that cuts across submodule boundaries needs as many MRs as boundaries it crosses, conflicting submodule pointer updates in the main require additional MRs (in the submodules) to resolve and coordination between those MRs.

They're basically fine for slowly-moving dependencies, vendoring, etc. but they emphatically do not solve the large-org many-team coordination problems that monorepos are meant to solve.

FWIW, git is a great monorepo platform for 1-10m lines of code (Linux, $MY_JOB, ...). It's only the very largest scales (Windows, Google3, ...) or asset heavy cases (ML, game dev) that need special treatment.