The UX that sucks is around what happens if they are unclean/how to update them. A checkout doesn't recursively checkout the relevant submodules. This is the biggest pain point for most orgs I've worked at. It's an easy setting to set (`git config --global submodule.recurse true`) but the fact that it's not default hurts.

Most engineers have a poor understanding of Git. My university had a great history of version control course right at the dawn of the git era (In 2006! RIT really speedran it, standardizing on Git by the end of the year, but also including tutorials on RCS, CVS, and SVN and a brief foray into Perforce). Still, a ton of my classmates just didn't get it.

The other major blocker is what to do with an unclean submodule repo; I honestly don't remember what git does by default, because it's bad. And most projects get unclean real quick. Makefile hygeine is not common, and for most of time most projects became unclean from a simple `make`. It's better now, but not great.