I think the issue comes about more when you have refactoring going on at the same time, and suddenly multiple feature branch maintainers need to figure out if they're going to merge in the main branch, rebase onto the main branch, if it makes sense to squash their changes first before attempting either path, etc. If they get only partway through either one, it's difficult work to pause and resume, so that integration work is fundamentally hard to collaborate on or even really review.
And never mind maintaining an ongoing integration of multiple unmerged feature branches— perhaps that's more of a "front end" issue for GitLab and Github to solve, but the whole business of patchsets and the like is very much unsolved, and that's painfully obvious when you see Debian storing their quilt patches inside of of a git repo, rather than being able to leverage git's native capabilities to achieve those effects: https://salsa.debian.org/debian/netplan.io/-/tree/debian/0.1...
