Benefits: easy cross-component changes, repeatable builds Issues: huge repositories, lack of code ownership boundaries, complicated versioning for individual products/components
Of course people nowadays rarely use submodules to build a monorepo, but I believe they would like to if submodules would be transparent.
Git X-modules are at early stage and only syncs a ref with a directory now. We'll add an option to pin a particular commit and most interesting - an option to receive module changes as PRs queue that could be individually accepted (and applied alongside with the necessary call site refactorings).
Git X-modules is available for GitHub users now, and we plan to add GitLab and Bitbucket support - both cloud and on-premises. So, we reuse platform's user management to only let repository admins/devs set up the modules - currently via web UI, and via .xmodules files in the future versions.