That does depend on what you think a "perfect" rating system is. In any game, it is possible to calculate the exact skill of a player precisely from few games. What is possible in all games is to calculate it to within a certain precision given a certain number of games.
The problem for diplomacy is two-fold. Firstly, the number of games played is generally fewer, so the precision must be lowered, and secondly, the nature of diplomacy is such that chance plays a greater role, thus, it would already take more games for the same precision.
As for riding coat tails in PPSC, according to the reward system, that is "good play". Thus the problem there isn't so much with rating system (be it points or G-R) but with what you define as good play, by the reward system.
To compare MM and Rait by another metric, % games won or drawn:
MM: 65.1%
Rait: 62.3%
However, MM has more draws than Rait for the number of wins.