Interestingly, Polybius ends his histories just prior to the cycle of Optimates-Populares strife so well-documented by Appian in his "The Civil Wars." In Appian, we see the ultimate perversion of Polybius' assertion of the strength of the Roman constitution, as the wealthy and powerful aristocracy vacuums up more and more of the wealth and political power of the plebes, resulting in a top-heavy oligarchy which ultimately collapses into roughly 50 years of civil wars of varying intensities, and the military dictatorship of Augustus. Worth noting that that dictator (Emperor) style of government would persist for another 450 years, so maybe Polybius' assertion of the superiority of mixed government left out a few factors. Excellent review.
Excellent review.