Its not that it can't be buggy but rather that you can fix bugs once you find them. A proprietary manufacturer of a pacemaker wouldn't necessarily fix your bugs in a timely manner; you depend on his cooperation. He could be out of business, no longer support that model of pacemaker, argue that it isn't a bug or use any other reason not to fix your bug. If you have the source code, you are always free to fix the problem yourself or hire a specialist to fix the problem for you.