I worry if a single nation would be first to develop effective ICBM countermeasures, the whole MAD doctrine would collapse on the spot and we'd have a next world war on our hands.
that out of the way, it is regimes that don't show a real concern for their people that is the danger. the only hold on them is the vulnerability of those making the decision. once they show disregard for their own welfare is when everyone should be worried.
I doubt that these exist, "look, these people are crazy and suicidal and don't even care about their own children" is a common propaganda statement, with the clear purpose of de-humanising the enemy. Even the most ferocious dictators care about some of the people, and about their own cultural heritage, and the country they rule. Nobody wants to be wiped off the face of the earth.
The RVs themselves are small, hardened and move incredibly quickly and are also surrounded by a plasma shield during reentry which may interfere with targetting with a directed energy weapon.
It kept the peace for 50+ years. (Excluding all the "little" wars.)