Seems to mainly be useful for monorepos as currently designed. Or, to replace a long-lived feature/refactor branch.
I think the only thing I miss from GitLab was being able to make merge requests depend on other merge requests, even across repositories. So I could make a backend repository MR depend on a library repository MR, and even enable auto-merge that’d fire when the backend MR was reviewed and the dependency was also merged.
Whatbmakes you say that? Devs use stacked PRs in small and large repos today.