Yeah personally I feel that it's humans who are the unstable component, unable to keep to policies for millions of years, not the machines. Our weapons have immensely increased in power in the 20th century, and improvements are being researched constantly. The number of countries with nuclear capabilities is ever increasing. How do you prevent nuclear armageddon in such a world? "Don't nuke all of humanity" is a policy that most humans would agree with, but you only need a few who don't. If we want to be a civilization that survives for millions of years, we have to give up control to machines that can uphold such policies for such time spans.