This is just branching, except your branch is in a different repo. You just call the branch downstream/master and your local master, instead of the more common origin/yourbranch and your local yourbranch.
For the rest the merging issues are identical. You can still have a merge conflict between downstream/master and origin/master if someone merged something conflicting into origin/master since the fork happened.
What you say is certainly true, but there are still meaningful differences: named branches give you the option to push them before merging as an informal remote backup (good) switch to a different named branch (neutral) and let the old one grow stale (very bad)
For the rest the merging issues are identical. You can still have a merge conflict between downstream/master and origin/master if someone merged something conflicting into origin/master since the fork happened.