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The VCS has nothing to do with it. Code has all sorts of subtle dependencies. When changes are made, other things are liable to break. When they're in someone else's fork, the person making the change is obviously not going to make sure that fork will stay working.

Either the change is accepted upstream meaning that everyone else making changes to the mainline needs to keep it working, or it isn't and they don't.

It's not about the VCS, it's about API compatibility being broken.

If your code is out-of-tree, every time someone changes an API that break your build, it's your responsibility to update your for and fix it.

If your code is mainline, the people who change API and break the compilation are on the hook to fix it.

Maintaining a fork is not difficult because we haven't invented the right VCS.