When one of these "modern, established democracies" manages to maintain a stable government (or even survive) for as long as the United States has, perhaps that statement could be made.
https://en.wikipedia.org/wiki/Proportional_representation#Li...
Other than perhaps the Scandinavian countries, that's not exactly a list that makes one optimistic about stability.
Edit: "Constitutional provisions for Presidential rule-by-decree had a lot to do with that, PR -- or even the particular form of PR used by the Weimar Republic -- well, if you want to argue that that was responsible for something, then go ahead and make that argument."
See my response to the other poster below. The PR system (and the myriad of squabbling parties produced by it) led to there being no effective government for two subsequent parliamentary elections.