There's plenty of exceptions. PR China just after Deng was a good exception, and so was the Soviet Union after Stalin. But they were only exception in the lam

There's plenty of exceptions. PR China just after Deng was a good exception, and so was the Soviet Union after Stalin. But they were only exception in the lame technical sense that you didn't have a single 'guy at the top' in charge, but a sharing of power by eg something like the Politburo.