The very same type of mistakes happens in file systems even without URIs being involved. Directory traversal checks look simple but sooner or later need hard-to-understand symlink following rules. Enforcing processor policy has terrible portability there (it even only became practical on Linux with landlock), but nonetheless I think it's preferable.
Not mixing URI parsers is a good advice for when processor policies are unavailable – but let's try to make them available more often.