In practice, a lot of the stuff that was difficult about branching and merging was just handled outside the VCS, and projects had rules about what you'd do inside the VCS. It worked, and still does for OpenBSD.

At Arbor, the last CVS job I worked (back in ~2003), we did release and dev branches no problem. I don't remember fretting about it much.

The big issue I remember is, it was a much bigger deal not to break the build.